Depends a lot what you're into.
For PvP:
Ragnarok Online private servers.
Really can't be beat for a straight MMORPG PVP experience. The games PvP system is amazing. Character customization via stat builds, skill builds, and a crapton of equipment+cards makes for tons of variety. Though i'll say right now, you could have god-tier equips, and still lose in a fight if you don't have the skills to win against a more skilled opponent. It isn't WoW. You don't have spell rotations, and your equipment doesn't equal your skill. You need to actually be good at the game, and know what the hell you're doing before you can survive in PvP.
The game is fairly easy to learn, and very hard to master, due to the shear amount of knowledge you need to memorize.
Also there is a good variety in classes.
http://iro.ragnarokonline.com/game/jobintro.asp
For cute things:
Aika Online.
Overall it's a fairly decent MMO. You level with quest grinding. It can be a bit of a pain to level up at times. But the combat is fluid and the PvP isn't bad.
The biggest plus for Aika is its pet system. Every character eventually gets a Pran.
it stars as a fairy, but once you level it up a bit it can transform into a little girl.
I swear to god they are the CUTEST things i've ever seen. I played the game just to level my Pran.
Not to mention later-game you can have it get older, and it will take on certain personalities if you talk to it certain ways during conversations. My favorite is Tough I believe it's called, your Pran will every so often pelvic thrust in front of a monster you're fighting. It's hilarious.
For a unique experience:
Shin Megami Tensei: Imagine Online
It's incredibly unique and in-depth. It's really hard for me to explain it fully, but it's damn fun.
The best part is of course, demons. You can capture, raise, and fuse demons. Like the usual SMT games. Your demon is a HUGE part of a battle, so you definitely need to keep up with it. It's very fun. Unfortunately it does suffer from being F2P. To level up at a realistic speed, you pretty much need to buy cash shop items to increase exp rates.
S4 League: Third person shooter, and one of the best you'll find. Cash shop helps a lot though, and it's expensive as fuck. No perma items from what I remember. So the game can get a little "pay2win"ish.
I'd suggest more, but I work better suggesting more specific MMO's based on what a person likes.
I've played damn near any English-friendly MMO you can name (excluding the bottom-of-the-barrel MMO's, though even then...i've played a few of those).
I'm a hardcore MMO gamer at the best of times, so i'm pretty good in this area.
When I get bored, I scavenge for new experiences via MMO's, and tend to have around 10 downloading at once at peak boredom.