I'm still trying to figure out who I want to play with full time and get good with, so I basically suck for now.  But I did noticed a shit load of Eddy and Christina players out there.  Followed shortly by Jin and Kazuya.  So I guess those are the four most popular characters of the player base? 
 
Also, I know that's more a knock on my own skills when I complain about an Eddy character doing the same (cheap) combo over and over again, but damn.. it's annoying. 

If you play on the PS3.. there are far more variation of characters online.   Which is why I switched to PS3.  You will encounter very good, Ling, Yoshimitsu, Baek, Kuma/Panda, Rojer, Nina-Anna.  Sure there are Eddy scrubs out there but for the most part, you will encounter all characters instead of the same ones on the 360.