Depends on where you are in the country. My tastes run to imperial stouts, imperial IPAs and barleywines. Of course, any good barleywine is going to be best aged a few years, so any barleywine you buy now I wouldn't recommend drinking until your 23 or 24th bday. That said, it's never too early to start some barleywines aging, so I'd recommend picking up a couple of barleywines NOW to start the process. If I know where you live, I can make specific recommendations...
That said, as for something you can have today, North Coast's Old Rasputin' is a solid imperial stout that is pretty widely available, as is Thirsty Dog's Siberian Night. I'm personally partial to Bell's Expedition Stout and Founder's Breakfast Stout as even better options, but they can be harder to find, depending on where you are.
Imperial IPAs... Weyerbacker's Double Simcoe is solid and in wide distribution. I don't love much of what comes out of Flying Dog, but their Double Dog is a pretty good choice. If you have Founder's, then Double Trouble is excellent.
All that said, I'd recommend jumping on beeradvocate.com or ratebeer.com and looking for reviews of the best beer store in your area. Any good beer store should have staff that can make recommendations on the best brews available in your area.
I don't think any top beers are nationally available, so knowing your local options -- and getting to know the staff at your local store -- is your best bet. Many of the most intriguing brews are small batch seasonal beers, and the best way to get ahold of them is to get to know your local staff, so they can set things aside for you.
Happy birthday, and bottom's up.