1955 265-V8 Chevy Game changer there.
1988-96 C/K. Changed how everyone looked at trucks. They road better, drove better and got better fuel economy than half the cars in their lineup at the time and paved the way for hte SUV craze.
C5 Corvette. I don't think I even need to explain this one.... it's pretty self evident. GM became SERIOUS about performance in late 1996.
CobaltSS/Ion redline. Turbo or Super. They put the power back into compact FWD cars, kept the prices in check and were faster than snot sliding out of a sick kindergarteners nose in the middle of flu season.
73-91 Suburbans. This, IMO, created the SUV craze that we are still dealing with. TONS of room, lots of people hauling, super tough. What else at the time could you haul 9 people with all their luggage for 2 weeks while pulling a camper/racecar/boat?
94-96 Impala SS. OK, it was a bit of a compromise on an old chassis dating back 2 decades.... but damnit if it wasn't cool as hell. Big car, decent handling, good power.... Perfect.
04-06 GTO. Brought RWD back for GM. Really good chassis mated with the fantastic LSx, IRS and made all the right sounds.
CTS-V. Take an Impala SS, give it twice as much power, better handling bigger brakes and a manual trans. Don't like it? You're a communist Bob Costas. Period. It's a slice of American pie served up by a hot bikini clad waitress wearing a 1911 on her hip and a tattoo of the american flag on her right boob.
2001-up HD trucks Duramax/Allison combo. This started the "Torque Wars" for domestic diesel pickups. Such a great combo. May no longer being the biggest baddest kid on the block, but it's still strong.
W-body.... I know I know.... everyone loathes these cars.Impala/88/GrandPrix/Regal. In reality they are super reliable cars. the Series II 3.8's were a great drivetrain in those. good power, 30mpg highway and durable.
C6. Takes the C5 to a whole new level of serious performance vehicles.