League of Legends for sure. If you haven't played MOBAs before, League will be the easiest to learn. That being said, League is still pretty hard to get your head around without any prior MOBA experience.
HoN and Dota 2 are pretty much the same (since HoN was initially a carbon copy of DotA). You will notice that HoN has a faster pace (since it lacks turn rates and cast animations). But since I like the way Valve handles Dota 2, I personally prefer it over HoN. So if you want a challenge, Dota 2 or HoN would be the way to go. I have a beginners guide to Dota 2 if you are interested.
LoL - easy to learn, hard to master.
Dota 2/HoN - hard to learn, hard to master.