i like the GT-ONE, it's faster than the other two at hammering the gas for long endurance races, or stuff you really don't have the time or will power to complete. however, that's not the attitude you should have in this game: it'll suck all the fun out of it. if it's fun you're looking for, the R390 is better. it's neither as easy to drive, nor as powerful (or cool-looking). still, though, it's mad fast, and only really useful in a competitive sense in LeMans or otherwise difficult races. the Viper GTS-R is really the best of the pack - it's the most eligible for car-specific events, the hardest to master, and really the most useful when it comes to normal, lower-level driving.
all 3 of them are great cars.