Changed up category names a bit. One thing to recognize is that the current meta revolves almost entirely around the two most OP heroes right now - Archer and Plague. Lines are built around their damage capabilities and you must construct lines that are capable of handling them.
This is the reason why you see Horse/Bro lines coupled with Plague and/or Archer - in the past, a Horse Bro line would have revolved around an attempt to time you out (which Bro did very well before they nerfed his orange). Now, Horse/Bro lines are simply there so that their Archer can kill you before your Archer can kill them. With that said, here you go:
1) Popular in the current meta (one might say "absolutely necessary")
- Archer - totes OP
- Plague - totes OP
- Hydra - works well with Plague
- Horse - protects from Medusa, shield gives a nice buffer against damage
- Wee - the only thing that stops her from being totes OP is she's one step down from Archer/Plague because she actually needs to white to kill everyone (compared to basic attacks)
- Medusa - she has definitely lost some luster due to horse, but she can still control a fight and she's absolutely necessary in GW
- Wolf - I user her a lot against Archer teams tbh. Still very good, and absolutely necessary in GW
- Moon - still very good. A rolling Moon = win against any team
- Bone Dragon - has lost some luster because of Horse, but also because of the reduction in Medusas. Still, nice to use against SE Spectrals, Spiders, Moons teams, and the occasional Medusa you still run into
- Plant - still very popular, can cost you some fights in FP if she times you out. Good in GW (on defense) as well
- Spectral - part of me wants to put him in the next tier, but you can't deny how popular he has become given the prevalence of Archer and Plague. Also, SE on him is suuuuuper annoying
- Crimson Witch - I still use her in almost every Coli lineup, a lot of times I wish I could use her in two lines. Also nice that you don't need to have her at a certain promotion level to be useful.
2) Has room in the current meta (useful for filling in the spots around the guys from #1 OR against very specific lineups):
- Bro - I waffled between group 1 and 2, but I just can't put him in group 1. He's only useful when paired with Horse and that only works because Plague and Archer are currently broken.
- Thief - maybe should be in group 1, but you don't build teams around/for/against her anymore. She can still get you a win via lucky purple proc in almost any mode
- Ninja - he falls over dead against Plague and Archer teams, but I still use mine as part of a burst group against Moon teams (weird to call them Moon teams now instead of Dusa teams ... but it's Moon that's the threat now, regardless of who she is paired with)
- Bard - similar to Ninja, honestly see very few of them these days
- Kaiju - was a decent counter to the Horse/Bro/Plague teams before Archer got her orange ... now? It's harder to use him. I still like pairing him with Spider though
- Spider - still very useful against tasty heroes. While Archer is tasty, the prevalence of BD/Plague limits her usefulness and places her here, rather than in group 1.
- Kraken - the anti-witch, but I think overall a bit less useful because (in my mind), Witch is winning you fights before her buff wears off while Kraken is moreso just slowing teams down to give you a better chance. Like Witch, doesn't need a certain promotion level to be useful
- Dungeon Man - solely for the SE. Dman + Spectral can = instant possession which is tough to deal with. Also helps getting your Dusa/Wolf/Moon/Wee started ... if the buff worked on more than the first line, I'd probably put him in the first group
- Shaman - oh how the mighty have fallen. I feel like I so rarely use mine these days, I was tempted to even drop him down another level. Still, he can be useful for helping your Kaiju/Spider/Spectral/Moon/Wolf/Dusa get going, especially against BD teams
- Idol - seems to be a somewhat popular inclusion with Hydra/Plague teams; protects against Spectral solos and has nice synergy with Hydra (also solidly mucks up Kaiju counters)
- Water Wyrm - I honestly use mine very often against Archer teams and it works very well. Well, I'm not sure if it's specifically Water or the other combo of heroes, but... yea.
3) Not totally awful, but really no room in the current meta:
- Silent Spirit - the current meta is not friendly to tanks. I still love my Spirit (I wish I had gotten that clown costume! ), but I very rarely use him. Not seen elsewhere very often either.
- Storm Drake - you know, he's actually quite good. He just doesn't warrant inclusion in most lineups over the people in group 1 and 2. Would love to use him more. (yes, I know Storm is a she, leave me alone)
- Understudy - still decent. Meta has left her behind
- Corgi - you know, at O6, she's really good. Maybe even burstier and tougher to kill than Ninja. I still use my Ninja more though, because the energy drain is very useful.
- Trio - still decent. Meta left them behind.
- Cutie - a decent tank. But: see Silent Spirit
- Rev - Man, I almost wanted to put him in the next group. The white can be a game changer, the green is a nice nuke, but ... stealing HP is a big no-no. Also, who would you include him over in the current meta? No room.
- Dracul - I also wanted to put him in the next group. Kit sounds great on paper ... I haven't used him in so long though. I have an irrational hatred of him.
- Cat Knight - On the flip side, I almost included him in group 2. His attack speed dmg buff is team-wide which ... honestly is very good in the current meta
- Mistress Manicure - this is what happens when heroes are properly balanced. Nobody uses them because they aren't ridiculously OP
- Spikey - if he had gotten a conservation item at O6, maybe he gets bumped up a group. Really curious to see what they do for his orange
- Roller - I waffled between here and 4. She was actually decent before Plague and Archer showed up. Not a game changer, but useable.
4) Pretty bad in and of themselves, I personally would not waste resources on them:
- Pixie - RIP Pixie. I will never forgive PB for nerfing healing in the same patch they released the perfect counter to Pixie (Kraken). They also did the same thing to Bro (Elf)
- Mega - he needs a good orange to be relevant again. Currently he's only useful for catching bombs in Temples
- Orc Monk - his orange was disappointing when it was first released and neither it nor he have aged well
- Snap - Currently he's only useful for pumping up your total power
- Dragon Lady - see: Snap
- Rotbeard - another hero whose kit sounds pretty good on paper but just doesn't work in practice
- Yeti - man, it saddens me to move him down here. With Storm finally getting a useful orange, my all-lightning (sans tank, oh well) team finally had a chance! But Plague/Archer pushed too many useful heroes out. He should probably be in group 3, but I don't think you should invest in him
- Skeleton King - his orange would have ruled the world back in the day. Unfortunately, we are no longer back in the day.
- Groovy - I know, you'd think the prevalent Horse meta would warrant her being higher ... she's just too slow. Maybe she's worth it for GW, but I think there's better bang for your buck heroes for that.
- Elf - see: Groovy. Also he's bugged and doesn't break shields like he should (I will say he's better in GW than Groovy because he's a control hero)
- Frosty - the Tenacity boost was nice, but that doesn't change the fact that the rest of his kit is too far behind :o
- Centaur - I like the idea of the orange, but waiting for an enemy to die is too long. He probably needed a full rework (or partial?) like Hydra had
- Genie - He should probably be in group 3 but I cannot stand him (the other hero I irrationally hate). He was really easy to counter with Ninja like 6 promotions ago and that hasn't gotten better. Another one I wonder about the orange with
- Rabid - such a bad orange skill. Ruined my hope that they would return him to glory. I actually wonder if they had never nerfed his energy gain if he would be relevant now or still just hopelessly behind.
- Zombie - he is good at eating crabs in GW
- Magic - he enjoyed a brief renaissance when BD was first released because BD triggers Magic very quickly. Unfortunately, Magic also triggers Medusa/Wolf/Moon very quickly and that was the end of that experiment (though I wonder if at a high enough gear level his damage gets high enough that he can act as a faster charging Wee).
- Faith - boy we are really reaching the bottom of the barrel. Maybe I should have made a 5th category called, "Only invest in these if you are a whale and working on your Total Power rank)
- Aqua - he had a miniature version of Magic's renaissance but he still wasn't very good
- Banshee - I wanted her to be good so badly. Immune to control effects? Seems like it would have been great during the Dusa era! Unfortunately it wasn't, and it hasn't gotten any better since. Also, I like her doll skin.
- Satyr - her orange skill wasn't enough to make up for the meta. Also, if my 21+ second Dusa stun is lasting 2 seconds against Bards and Ninjas at O6, how long does her 0.5 second stun last? Are they simply immune?
- Dust - you know, I know there are a few guilds on my server who use Dust a lot in GW. I still don't think it's worth the investment :o
- Assassin - see: Faith
- Vulture - was never really very popular, even during the control era when you thought he would have had the greatest effect. If you squint your eyes really hard, you can envision a scenario where Plague/archer kills someone on your team and he swoops down and 1-shots them. Don't give yourself a headache though, it's not worth it
- Pole - see: Faith
- Vile - I have a guildmate who built him and insists on using him just because of the resource investment. I've also heard that he's popular on rune-less servers. For me though? Not worth it. And for you? Also not worth it.
- Dopple - such an interesting kit. Sad that it never got to be useful. Maybe an orange would fix that?
- Were - tanks, man. I actually think him + Wyrm's magic vamp would be an interesting combo, but it would probably just be an interesting way to lose.
After writing that up, I may go back through and break up the groups a bit more; at least the last one. There's a very real difference between investing in someone like Faith Healer (already has an orange, not at all useful) and someone like Vile or Were (neither have an orange, both can be maybe a little useful given the right circumstances). A hero without an orange still has a hope to become useful (see: BD, Archer). A hero with an orange who is still not useful is doomed to your dustbin.