Of course, I'm going to say '06-'07 CTS-V...
I would offer that - as a newer DD - the '10-'13 Camaro might be a smart pick, based on your wants/needs. Seems like people that own them enjoy them. I'd like to drive one someday...
The GTO was the go-to, GM-focused V8 RWD 4-seater - but the Camaro has proven over the last 5 years to be a great platform. Looks like some nice ones under $20k. Easier/cheaper to finance too if you're going that angle.
Crazy thought, but would you consider the V6 Camaro? 305hp, 30mpg...
With my 4cyl BMW, I've come to appreciate that I'd rather have a stick-shift and an engaging platform to drive over stonking power and a slushbox automatic.
Do you absolutely need 4 seats? A low-mile C5 Corvette would be a very reliable, fun-to-drive platform, and the rear hatch holds over 21cu.ft. of stuff (more than a modern Buick sedan).