Both '97 R390s.
The burgundy one doesn't need suspension changes at 351hp (yes, the stock value can murder the competition), just slight LSD changes, all other non-engine modifications. The blue '98 "Special" is heavier, only slightly more powerful, understeers more, and costs you 1 million credits.
The black-and-red one is tough to win (damn GV enduro!), but I've made better laps with it compared to the '98 (light blue one). The reason? It's 550 lbs lighter than the '98, even though it gives up 30hp.
Bah, I've never noticed a differenc between the '99 "Marlboro" GT-One, or the '99 "Flying Lizard" version. Both are the closest thing to an F1 car there is in the game. There's only one road version, and it understeers too much.