I adore many, many different pairings, both to read and write -- and the rarer, the more unconventional or uncanonical, the better. In Prison Break, Michael/Alex is the pairing of my heart, but I would read Alex/Lincoln, Michael/Gretchen or Alex/Lincoln/Sara in a heartbeat. (Seriously, Alex/Linc/Sara? I would be ALL OVER THAT).
I think this comes from my default lens, which is 'It's perfectly possible that X is attracted to Y' where X is absolutely anyone and Y is absolutely anyone else. (As I commented, my lens is clearly like that of a fly, with multiple little windows for each and every combination *g*)
I am completely prepared to believe, without needing any textual support whatsoever, that any given characters will fancy the arse off each other -- regardless of gender, race, age, species, position, usual orientation, past history, relationship, whatever. It doesn't matter whether they're on different sides, whether they canonically hate each other, whether they're related, in love with other people or whether they've never even met, this lens lets me see an attraction between them.
It's why slash is so natural -- why I immediately ship Alicia/Kalinda despite Alicia being absolutely *defined* as The Good Wife, why my first thought on the latest Survivors is 'ooh, of course, Tom/Greg'. It's why actual sibcest (Sam/Dean, River/Simon) or faux-cest (Buffy/Giles, John Connor/Cameron) doesn't squick me, why I like Willow/Giles at both ends of the power imbalance dynamic: S1 Willow as barely-legal naive schoolgirl and futurefic Willow as all-powerful witch-goddess.
Ray Carling hates Sam Tyler, Mulder hates Krycek, the Master hates the Doctor -- I'm still quite happy with the idea that they can also, at some level, *want* them. Rhys absolutely and thoroughly loves Gwen, but that doesn't mean he can't also want Jack. Buffy never met Gunn, but that doesn't mean she wouldn't want him if she did.
What I really enjoy about unconventional pairings is the context. Suspension of disbelief is pre-programmed, so I approach it from the point of 'okay, obviously they would fancy each other -- now, what would make them act on it?' This is, I think, why I love first times and Aliens Made Them Do It, but not established relationship stories or pure PWP -- because in those, the 'how/why/when it happened' is irrelevant, and that's my favourite bit.
There are pairings that don't really move me, but I don't think there are many that I couldn't, in some way, shape or form, manage to *see.* I guess I'm just easy that way :-)