The three seasonals that I always look forward to are Bock, Octoberfests, and Christmas/Winter beers. Sam Adams always hits the mark with the first two, but I have to agree with the others here about Old Fezzy.
The first couple of years that I had it, I was amazed. It was incredible, and I swear that they sold it seperately in bombers. The last couple of years though, I just haven't been impressed.
Anchor Steam Christmas Ale is the same way. I have had it almost every year for the past...25-30?? years. Sometimes it's a hit, other times, not so much, but it never disappoints.