Stone IRS is definitely up there with the best. I have a 2008 Release that I need to drink this weekend.
Goose Island Bourbon County is not only my favorite Imperial Stout but my favorite beer of all time.
Old Rasputin is too sharp tasting and lacks a lot of stuff I look for. It's better when it's old though.
If you enjoy coffee infused Impy Stouts, you can't go wrong with Founder Breakfast Stout, Peche Mortel, and ESPECIALLY Alesmith Speedway Stout.
Other crackers:
Any Great Divide Yeti variant
Oskar Blues TEN FIDY (liquid tar from a can)
Deschutes The Abyss
Bear Republic Big Bear Black Stout (Bear Republic = best bargain bombers ever)
Smuttynose Imperial Stout
Samuel Smith Imperial Stout (smoooth)
Victory Storm King
Founders Imperial Stout (should be coming out soon)
Thirsty Dog Siberian Night
...and many, many more. Craft brew has become so "hip" these days, I can't even keep up.
I just bought this local brew Wormtown Norm I gotta try. Oatmeal stout brewed with Cacao Nibs and Coconut. Should be slammin'.