I liked Ken until HDR came out. After a few rounds, I got tired of seeing him so I moved to Dee Jay.
I mainly use 'Gief in IV, with the occasional Dan and Sakura to mix things up. It gets annoying playing someone that jumps the whole match trying to avoid an SPD. Except Akumas, since their air fireballs are piss easy to avoid, they always telegraph the raging demon (from a full screen away), and they die quickly.