PvP.
Loved World of Warcraft for their battlegrounds/PvP.
Runescape was meh because to some extent PvP was good, back in the day of DDS/GS specs and ice barrage spamming + luring/teaming was pretty bad but with the EoC update PvP is pretty fun. 1vs1 fights on old school/legacy is also pretty good.
Order and Chaos (mobile WoW clone) was another I enjoyed for their PvP - having earned two seasonal PvP mounts on my main account and having to maintain a top position in order to keep them was something I really enjoyed, they are coming out with some really good updates so I'll probably get back into it.
I also enjoy having a ton of customization options, a lot of people poke fun at Runescape for being Fashionscape but at the end of the day if I'm going to be spending thousands of hours looking at my character, I want him to look good.