Firebird
Impala SS certainly. Even Chebby fans think it's slow, even though it would hand the 1994-1996 Caprice-based Impala SS its ass on a silver platter.
That's because Chevrolet people equate 'front-drive' with 'slow' which is why they suck.
The fact that the Buick Regal is (/was) an older design (I'm hard-pressed to dismiss a seven year old design as "old as ****"...

) also adds to the sleeper factor. Midsized cars of TODAY are fast, but they weren't nearly as fast in 1997, save for the Regal GS and Grand Prix GTP (which are both
still the fastest, save for an Altima 3.5SE with a manual transmission.
Agreed - which is why it was a sleeper in 1997. Hell, it was a sleeper in 2001. But based on the vigorous ad campaigns of midsize sedans today, everyone knows that today's midsize sedans are quick. And the vehicle is judged on today's scale, not 1997's, since we're here in today, not 1997. Making the Regal GS just another midsize sedan with quite a bit of power.
A seven-year-old design is what I refer to as 'scary old' in the midsize sedan market, particularly today's market since the trends are fluctuating more than ever before (power, hybrids, interior and exterior design, luxury features). Few designs of that age remain relevant in any market, though there are several glaring exceptions, namely in the light truck market and from Jeep. Trutufully, I think the Cherokee is among the best-looking vehicles of all-time, and while I really like the Liberty, I love the Cherokee.
I saw a Briarwood yesterday; I was really proud of myself. Obviously the Wrangler remains and will always remain relevant and is the best example of 'minimum investment, maximum returns' in the history of economics, as I've discussed previously.
I don't know why there are people who consider the Pontiac Grand Prix a midsize sedan, but evidently those people do indeed exist, and even more evidently, I've found one of them.