⚠️ This is not a Tier List, as they are subjective, and many opinions may differ. There are players that will tell you that any hero is playable just fine, and to some extent, it is true.
The Hero Difficulty list tells you, how difficult is the hero itself in Horde Mode. As the mode name might reveal, you are fighting groups/waves of enemies, dealing AoE damage is the key, however some heroes were not designed for this, since vanilla Dota 2 is a PvP game, focused mostly on hero fights.
Ultimately, how you play and build your hero will decide about your performance, not this hero list.