Dungeon Fighter Online, got shut down, but is now coming back in 2015 with (hopefully) better care than Nexon could ever give it.
It's really action packed, fast paced, and combo-oriented. The game has tons of classes to choose from, usually four paths per class. I love PvP, and this is the exact reason I decided to pick up this game in the first place, was for the PvP solely, but I did play some PvE as well. I had to level up my characters after well. My main was a Monk, which was about level 50+. Priest classes are usually heavy characters, but the Monk was really agile and has really good combo's.
Here's an example of DFO PvP, if you see a buff guy with blue glowing hands, that's a Monk. Anyone who hasn't played the game will likely not be able to keep up, LOL.
Disclaimer, this is high level play, and I don't think this is "fair arena", where it's more balanced, but it's sick nonetheless:
https://www.youtube.com/watch?v=qDHFWlfTqGY
After the shutdown, I really wanted to play Dragon Nest, but I don't have a computer to support it. It requires a PC, and I don't have a decent enough PC to run it, except the same PC that I played DFO on, because DFO only requires the specs of a potato.
It does look really fun though, wish I could play it. :/
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On another note, there's also Super Smash Bros, which is one of my favorite games of all time. I love playing it competitively online. I'm currently playing Smash 4 if anyone wants to go some matches, I have a killer Greninja. c: