Coming from someone who placed well at EVO this year (I'm just taking the last version of each game) :
SF2X : Old Ken > any version of ryu. But Akuma is stronger than the two of them.
SFA3 : V-Ryu > any version of Ken. But V-ism Akuma is stronger than the two of them.
SF3:3 Ken is just much better overall. But Akuma is more dangerous.
Ultrastreetfighter4 : Ken is stronger, Ryu is trash now. Akuma is top3 no doubt, and better than ken.
Answer : Akuma.