I would probably give the Supra the nod over the Viper, but not by much. Both are competent, high-powered FR machines, but the Supra is just a little more my style.
I haven't messed around with the Esprit GT1 as much, but it's also nice car once you get used to it.
The Tigra Ice Racer is an awesome rally car. It doesn't get a lot of press, but 524hp @ 2100lbs. is a killer rally car and can beat the best AI cars on the track, too.
The Calibra Touring Car is one of those perfect, can't-spin-it-if-you-tried type of cars, which is nice at times, but a little boring. Great-looking car, though.
For only 6 cars, Opel has almost all the bases covered in GT2; one econo car, a sedan, a compact car, and a rally and track monster.