There are a lot of heroes where, if I see them in FP, I know I'll win the fight with my normal go-to team. This is on S4 with runes.
Trivially easy to beat: Centaur, Goblins, Revenant, Snap, Skeleton, Dopple, Healer, Polemaster, Dragon Lady, Assassin, Banshee, Zombie, Dust Devil
Easy to beat: Satyr, Orc, Unicorgi, Genie, Magic Dragon, Druid, Rabid, Roller, Vile Bile, Rot Beard, Vulture, Water Wyrm, Weredragon, Frosty, Kaiju, Mainicure
Only slightly harder: Drakul, Catapult, Understudy, Aqua, Pixie, Idol
Some of these heroes are still useful in specific situations. For example, all the damage dealers (even Centaur) can be useful in War on offense, because they fire powerful whites right away. And some can be okay in Colosseum -- for example, if I need Ninja to beat two lines, I might consider using Goblins for one of them.
So most useless hero overall? I'm still not sure. I really don't like Revenant, Assassin and Banshee, so maybe those would be my bottom three. Yeah, I know a lot of people still use Revenant, but I find he not only dies quickly (when I'm attacking him), but he makes his whole team die faster by stealing their health. The only thing going for him is the bug where he can become untargetable for a few seconds after using his white.