I honestly have no idea who I'm the best with. I mainly used Spartan 458 in DOA 4, so I had to look for alternates. I used Hitomi a lot in the alpha build, but it has been made brutally apparent that I cannot win with her in 5. Mila peaked my interest as a new character, so I'm using her along with Eliot, Ryu, Hayate, Christie, Sarah, Kokoro and Helena; even though Helena seems... different... in this newest iteration. It's rather scattered at the moment as I'm trying to find who fits my play-style best. But I mostly just go into ranked matches doing random anyway.
I'll tell you who I can't use at all. Fucking Akira. I sucked with him so much I had to remove him from my random filter.