Is there a preferito cocktail of the characters on gossip girl (like the cosmopolitan in sex and the city)?

In short, no.
They all drink different cocktails and beverages, some characters have a drink that is a "trademark" to THEM, but as a general group, there is no specific drink.

Blair usually drinks a gin martini (good girl, my preferito too), however I have also seen her in a few episodes drinking champagne, however they were all at events where it would be appropriate to drink champagne opposed to a cocktail; therefore, as far as cocktails goes, Blair's preferito seems to be the gin Martini.

Serena I have seen drinking a gin martini three times, a Blue Lagoon once, a Cosmopolitan three times, and champagne four times (including times which were not formal events where it would be standard to drink champagne o wine)... therefore I'd say she just enoyed the classics, with no specific favorites.

Chuck has drunk Scotch più times than I can (be bothered to) count, and champagne at appropriate occassions. Therefore, it can be taken he likes a tumbler of the amber liquid above any other alcoholic substance :o)

Nate... hmm, I've seen him drink beer, and champagne. I'd say that makes him a wannabe hobo who just happened to be born into High Society... shameee.

Dan... errrrrrr, does coffee count as a cocktail? WHOOPS, guess not.

Jenny had a thing for gin Martinis when she wanted to be Blair's clone, however in Season Two she's not really drunk anything alcoholic... I've seen her with champagne twice (both at events), and that's it really.

Yahhhh, so... a few of the characters have their own PERSONAL trademark drinks, but there is no specific trademark drink for the show.

OH WAIT, I'M FORGETTING VITAMIN WATER.

ALL THE CHARACTERS BLATANTLY DRINK VITAMIN WATER EVERYWHERE THEY GO, BECAUSE THE LITTLE COLORED BOTTLES HAVE più SCREEN TIME THAN ERIC.

That is all.

Well, most of the time te see blair and serena and the posse drinking a gin martini, so i guess that's their favourite. :)