Anymore when I visit a brewery, I'm really looking for taproom exclusives. Here is my list of top ones I've been to. Any heavy hitters I'm missing? Bruery Jackie-Os Odells (may be a bit of homerism there, but they have typically some badass stuff) Kuhnhenn Cigar City (though being out-of-market I'm never totally sure what's taproom only)
Most of Russian River's beers are taproom exclusives. Lagunitas also usually has a number of experimental small batch beers where they vary the formula of one of their brews, like Hop Stoopid with Galaxy or Nighttime with Simcoe.
Firestone Walker Barrelworks...in Buelton....some of their best stuff is taproom only and extremely limited.
I came here to say this; so much stuff that's spent 7 years in barrels (Saucerful, DDBA60, Bravo 217...).
Hill Farmstead Allagash Captain Lawrence Are probably my favorites of the ones I've been to. Besides that I really would advocate for the local brewery (mine being Captain Lawrence).
Dogfishhead brewery and their several brewpub/grills for mid-Atlantic I'm sure wins hands down. They always have 5+ of "in house/on premises only" offerings that aren't released anywhere else.
Cigar City has a lot of brewery/brewpub only beers and a lot of them are quite good. The Peach American Sour Ale was awesome, about on par with Westbrook Gose.