Samuel Smith's OS was one of the first craft beers I ever had, about 15 years ago, and will always have a special place in my heart. It's probably the reason OS is my favorite style of beer.
Having said that, I recently tried
Founder's Breakfast Stout for the first time while back home in the States for a few weeks. It was love at first sip. I drank as much as possible while I was there, and want it to be my first attempt at a clone later this year. Soooo good!
Southern Tier's Imperial Oat (another recent discovery for me) is noteworthy as well, but maybe that doesn't count...