Sorry for the bit of a necro;

If you went by actual PvP power from the games, you're list would be very different.

BL1: Lilith obliterated any other character/builds in duels, because her skill. You could come out of it, one shot, have full damage immunity, etc. Mordercai, etc. the others could not compete, no one could. She was really OP. The shield bypass meant nothing to her damage reduction, and she had daze on shots/action skill. It still wasn't even fair pitting the other 3 against her alone in one of the arenas. She'd still crush everyone.

BL2: Surprisingly, Maya. Her skill in PvP would one-shot anyone. Wouldn't pick them up, etc. just insta-kill, but it was insta-kill. It was so fast to do, any weapon setup, build etc. (even for the Maya) was irrelevant. Just duel, hit F, done. Zero was/is not quick enough to escape it either. All she had to do was look at anyone and hit F and his skill left enough time with it's animation.

TPS: Nisha was actually the OP one, as auto-aim. This was probably the most 'fair' game to duel in, without clear instant-winner characters, but Nisha was still pretty close to Maya in 2, in that F was an "I win" button.

3, I don't know. Haven't dueled much at all yet, But I'd expect FL4K to be the one, as he's got invisibility, but it's not immunity like Lilith was, so who knows.