Probably the least-used Viper in the game is the GTS-R Concept that was in the online dealership a while back. It peaks over 900 hp, sounds great with the racing exhaust, and looks amazing in black (the stripes stay). It doesn't even need the racing transmission (and associated annoying noise) to be fairly competetive (235mph in 5th gear at the end of the SSR7 tunnel is fairly normal)
The various 350Zs might be decent, but definitely not in the Vette/Viper/Enzo/GT/whatever league. Good luck finding a decent match for it.
The Countach LP400 and Acura DN-X do around 210 out the tunnel. The DN-X doesn't have transmission whine with the adjustable box, so I used that.
The Plymouth Superbird has pretty good power and sounds amazing (too bad the FC trans is so noisy), but the power drops off sharply in the very upper end of the RPM range so the gearing has to be done more carefully than on other cars.
The Chevrolet SSR has good power, but the stock 4spd transmission harms acceleration. The torque somewhat makes up for that.
The Eagle Talon is a surprisingly quick little car, but the turbo lag and somewhat-lacking power keep it around the same league as the 350Zs (just as a quick estimate).