I really would struggle to pick definitive favourites as I'm a fickle bugger but here's a few nonetheless
Most William's Bros. beers - but especially Midnight Sun, a meaty porter with a lovely ginger glow, Seven Giraffes, a slightly deranged but incredibly refreshing lagery concoction, and probably my favourite of all, their ginger beer, a truly spectacular drinking experience, far better than the Crabbie's nonsence.
Other UK brewed beers would have to include Oxford Gold, Bitter and Twisted, the stupendous Schiehallion, and some of the Badger beers.
American beers are well worth an investigation - I'd never have believed until a recent trip over there, but there are some superb beers from the likes of the Flying Dog Brewery, Sierra Nevada (much better out of a bottle over here than on tap if you ask me), Anchor Steam, and the Stone Brewing Company (their Bastard ales are lovely and their Ruination has to be drunk to be believed)
Belgian beers..... I don't know where to start, far too many extraordinary beers, but Deus would have to be up there, an effervescent joy, and probably delirium tremens also.