there is a teeeensy chance this topic may have crept up before but i can neither confirm nor deny that
the F1 cars are the best at winning races with little effort...
imo the best race car is either the Honda S2000 LM Race Car or the Toyota Altezza LM Race Car but other will say R390 or GTOne race cars...its a matter of preference...
it depends on which race you want to win or whether you want a tough but rewarding challenge...you could try doing the Pro mode easily in an F1 car or try challenging the Amateur mode in a rally car...its more fun that way....