1x2/2x1 and 3x4/4x3 are the ur-pairings, with 13x5 being quite popular as well. And 13x6 or (for the kinky) 6x13 are faboo. 6x9 and 13x11 if you're into het, obviously. There are also quite a few permutations of 1x2x5.

(Can you unpack that or do you need a translation?)

There's also Heero/Relena, but I've seen very few fics with that pairing.

There needs to be far more 11x9/9x11 love in the world, if you ask me.