Tina for me. Best, main, favourite, everything. If it wasn't for her I'd never have looked twice at DOA1 in the first place, never would have picked up the series. She was the first female character in a fighting game to use a slower grappling style (predating even R.Mika, lots of people don't realise) and that was such a novelty at the time, I had to give it a go. So, using Tina since DOA1, yeah I'd say I am pretty good with her and it's no surprise she's my main by a long way. I think in DOA4 my character usage puts her at 96.something% and already in DOA5 I'm at somewhere over 80% usage with her.
That said, Helena's fighting style looked so unique and different as well when she came along in DOA2, I did almost switch over to her. But I've never been good with her, in fact whenever I use her I can only resort to brainless button-mashing. So, she's who I wish my main/best was.