Idk, but since it's past your birthday, you should make an effort to hit up every bar in town with your friends. There really aren't enough that you could never hit up all of them, so just check the places out and see what you like.
Generally I'm a big fan of Irish pubs, but when I was 21 - 23 I liked clubs. Or though I did, but looking back I really didn't, they were just the wildest atmosphere for being really drunk. Beyond that, pubs/irish pubs. Ri Ra's which is close-ish to a club environment on weekends, but not too much that it has sketchy people in it. Or Finnigan's pub, which is a sort of divey irish pub (looks like a pub inside, but not as decorative as most irish pubs) that has a nice (sometimes dismissive, but mostly chill?) usual crowd sprawled out across the bar as well as surrounding the pool table.
Esox is a super laid back 'pubby' looking bar. Packed on weekends, but chill during the week, usually. Also Mr. Mikes.. Not so much a big bar, but it gets a crowd on weekends and is generally pretty laid back.
However, if you're just turning 21, you probably just want to hit up most of the big bars like redsquare, nectars, rasputan's, RJ's, JP's (probably the most divey looking bar in burlington, but a big, fun crowd) lift (definitely the ghetto bar if there is one, but a good time sometimes, and probably the youngest crowd for a big bar in burlington)...
Just explore, 21 is damn young, so don't be afraid of the bigger places that people think are doucheyer, because there's a good chance whether you enjoy them now or not, you'll feel too old to be in there within four or five years. (at least the clubby ones).