I would say that the easiest character to play is my own; Alpha. I'm pretty sure Akira or Raidou are the hardest characters to master.
Free Step Dodge
deathofaninja
Chapstick
Akira hardest, Momiji easiest
Force_of_Nature
It's a toss up between Momiji & Rachel. Hayabusa, Brad Wong and Ayane are the hardest DOA characters.