Yeah Twi would be my ideal for a relationship,
rarity is way to high maintenance...but worth it...
Pinkie would be to tiring, great to hang out with now and then, but to tiring to be with all the time.
RD, Yeah massive personality clash there.
Fluttershy...d'awwwwwwwwwwww, but yeah she'd probably to timid to really enjoy a relationship, plus if she DID go for somepony, they'd probably be a outdoorsy, animal loving type, and I really don't like outdoorsing much.
AJ, again being in a realtionship with her would require helping out on the farm....no thanks, maybe once in a while like major harvesting time as a freind, but not all the time.
....wait we're supposed to be humanizing them first.......umm I mean yeah totally talking about humanized versions.
Build a fool proof system, and the world will create a better fool.