They make a mean margarita at Sandoval - Columbus between 72nd and 73rd. The menu is mexican, but not your standard just-melt-cheese-on-it-and-call-it-authentic fare. Unique empanadas and tamales. Cheaper, with a more festive atmosphere is Gabriella's (search earlier postings - pretty sure this place has been mentioned on this board) - two locations, 75th and Columbus and 93rd and Amsterdam. Citrus (76th? and Amsterdam) makes a bunch of kinds of margarita's and frozen drinks too, but I think the food is overpriced.
I don't think there is a general rule, all purpose answer out there, but since another respondent opined on Margaritas, one of my favorite libations, I would have to suggest Zarela, 2nd Avenue between 50th & 51st. Zarela Martinez insists that there's nothing in them but lime, triple sec and tequilla, but these are magnificent, lethal concoctions that are probably far less legal than that. Zarela is a lively, fun place with marvelous, inventive food and is just about the most affordable venue among our haunts.