Here's a list of some suggestions on good beers many have offered.
A great first beer to try is a Williams favorite: Milwaulkee's Best aka "The Beast". Excellent, full flavor--it's a real beer experience. Yet light enough to play Beirut or Flipcup with. Very well priced for such a great beer.
Try one of the Belgian offers that are usually available at Cole ave. and even spring street:
Hoegaarden Witbier (white beer) 5%alc.
Blanche de Bruges (white beer)
Duvel (golden ale, absolutely amazing) 10%alc.
Grimbergen Tripple (abbey/trappist ale)
Lindeman's Lambic ale (flavored with cherries, peaches, etc...it's a highly unique beverage)
Commentary on the above: Hoegaarden > Blanche de Bruges. Duvel and Grimbergen are tasty. Lindeman's tastes very different from normal beers, but there is no essential difference in the method of brewing. For a nice selection of Belgians, try the Beverage Den on Rt. 7 in Bennington. They've got Orval, Chimay, Rochefort, Witkap Pater, Cantillon Gueuze, Saison Dupont, de Ranke xx Bitter, and some other stuff. For cheaper belgian style beers, Allagash, Unibroue, and Ommegang are all awesome.
If you hate the Belgians (and, after all, who doesn't?), here are some other great brews (usually available at Spring st.):
Samuel Smith - the whole range: Oatmeal Stout, Imperial Stout, Taddy Porter, Pale Ale, Nut Brown Ale, Winter Welcome Ale.
Unibroue Le Fin du Monde
American Craftbrewed/Microbrewed Beers
Stone Imperial Stout
Otter Creek is absolutely fantastic. I especially recommend the Vermont Lager, but all varieties are very tasty.
BBC brews up some mean beers. Almost anything by them is great, except for the Steel Rail Extra Pale Ale. Support local beer! The BBC Imperial Stout is easily one of the best Imperial stouts you can get anywhere.
TrÃ¶egs Brewing Co. from PA brews a wonderful American Pale Ale.