I'm thinking the Impala you're referencing may be a '63 that had a 409-based 427 with aluminum front end and bumpers. Z-11 was the designation, I think. And they were built for drag racing and nascar. Or it could be the first Mark IV big block that was bounced out of nascar until it was fitted in a production car in '65.
The ZL-1, as Marv stated, is basically an aluminum version of the L-88 with second gen L-88 heads and a bit more camshaft (L-88 cam wasn't exactly mild, either).
I'd go with the Hurst Hemi Dart/Cuda as the quickest. Though a T-Bolt or SD Tempest would probably be right there, too. These cars weren't street cars, though, and were delivered sans warranty (or paint, in some cases).
Modern cars? Callaway Vettes could be ordered as regular option through Chev. dealers (B2K?). I think he made at least a couple or so twin-turbo ZR-1 cars near the end of the run (at least one Speedster and coupe) that are probably fairly quick. The "sledgehammer" he built went 254 mph or some crazy number. I think Lingenfelter did the driving, too. But this car was a one-off and probably not too emissions friendly, either.
Saleen S7 is probably the quickest current car. They're taking orders all day long at Saleen dealers. $375k a pop, I think. The Mosler's fast, too, and his best looking car to date, IMO - if he actually builds it. If so, it'd be a cool car to have.
Oh, and I wouldn't mind having a memory like Gene's or a wagon like Mark's.