It's a tricky question. Most of them would be impossible to date and have a romantic relationship with since they'd be too busy saving the world from bioterrorism... I mean, my obvious first choice would be Jill but realistically speaking it would be better to just have a quick fling with her and then leave. Lol.
The only main character which I feel would work for long term is honestly Rebecca. She's more of a scientist now and isn't out on the field, which would be easier to work around. She's far from my favorite character in the games but she'd probably make for a pretty sweet girlfriend.
If we include supporting characters, I'd totally have a fling or two with Moira.... mmhmm, wouldn't mind continuing the Burton lineage, am I right?
And lastly, I would like to have some private time with the three Dimitrescu daughters if possible. Hopefully I'd survive the night but if they finish me off by sucking out my blood I'd still die a happy man.