My vote goes for HoN, because I enjoy the gameplay, graphics and animations more than LoLs.
BUT, this should be taken to note in this kind of discussion;
For a more casual environment, LoL is the #1. (Game I've played the least, quit)
For a slightly more challenge of a game, and very teambiased game, HoN is the #1. (Game I've played the most, quit)
For the greater competitive experience, Dota 2 is the #1. (Game I'm playing at the moment)
For the classic game with the original genre of the above, Warcraft 3's DotA is the #1
A competition between the above is pretty much unfair. Why? Because they all recruit different audiences. (Of course there is a mix, but mainly)
Add to the equation that the games have different joiningconditions, and have been launched differently, which may have affected its startoff audience.
Also look at the companies behind them, and how they are able to affect the games.
Just throwing this out there