My top 5:
Rochefort 10
La Trappe / Koningshoeven Quadrupel
Ommengang Three Philosophers
Westvleteren 12
And just for a little bit of that hometown pride...
Yeungling Traditional Lager
Rochefort and Westvleteren are both Belgian Trappist breweries, and Koninshoeven is a Dutch Trappist brewery. All three are absolute masters of the Belgian style strong ale.
Ommengang is a US-based brewery that does a mighty fine Belgian style as well (seeing a theme here???)
Compared to those other four, Yeungling is absolute crap. However, it's a Pennyslvania tradition, and still reminds me of home, so I have a special place in my heart for it. A cold Lager on a hot summer day is mighty, mighty good.