Westvleteren 12.
I bought 2 bottles of Westvleteren 12 and some other good beers onto the X-mas market in Brussels (Belgium). There are also a few specialised beer stores in Brussels which carry the beer all year long. If you are around Belgium you should go.
Or go visit the Westvleteren Abbay yourself.
Easier to find is the Sint-Bernardus 12 it's in sale in almost every big grocery store in Belgium. It's a Belgium secret that the recipe is the same and people who tried them both will agree thats it's nearly impossible to taste the difference.