They are all good cars, and about equal, so go for looks and price. Do you like open cockpit or closed cockpit better? Do you mind dropping $4.5 mil on the Pescalaro (the fastest LMP in the game) or would you prefer free prize cars (C9 & Speed 8). All are roughly equal, and if price is a non-issue, go for the coolest looking one.
FWI: The Speed 8 is won from the Sarthe II enduro, and the C9 from the FGT series.