Cullen.
However, I share the view of other posters that my preference is probably mainly because Cullen is older and more experienced than Alistair, and so I am these days. I like that he's mid-30s, he's confident in his professional capabilities, he knows what we wants and he can be pretty hard arsed about getting it. I also love that he stands up to the inquisitor at the war table ("No. This is ridiculous"). I also like that he has some flaws but he openly admits his mistakes and works on them. He's by nature a conservative kind of guy, further jaded by some horrific experiences, but he sees the problems with the world, be empathizes with those hurt by it, and he is trying to be open minded and come up with a compromise. Sure, he's not very experienced in personal relationships, but I don't mind that either. He takes them seriously, like everything else he does, which is a plus in my book because you know he's in for the long haul.
True, the Cullen-relationship doesn't have the passion of Hawke-Anders. But Cullen is neither as needy nor as batshit insane as Anders. The Anders relationship is the kind of thing I love in games (especially when playing an equally crazy free-the-mages Hawke), but would flee from in real life. Too much DRAMA.
I still love Alistair. I adored every moment of that relationship, and he was the perfect partner for my young and idealistic warden. No doubt he grew into a lovely man (if not the world's greatest king) and I am sure that he and my lawful goody-two-shoes warden are very happy together (with her doing most of that annoying ruling stuff while he charms people and looks dashing). But during the play through they were do young that I can't really relate to that relationship like I can to the one with Cullen.
All that said, Garrus is still my number one Bioware boyfriend. I enjoyed watching Sheppard and Garrus go from an uneasy working relationship to close work colleagues to friends then good friends then best friends then lovers. It's always seemed both the most stable and the realistic of the Bioware romances - at least if you can overlook that he's a big blue dinosaur and they can poison each other with their bodily fluids.