Carrera GT would be my number 1. Great drive, it's quick but you have to be careful with it or it'll bite back, which it was notorious for IRL. This can be tuned out of it though, but I think keeping its IRL characteristics and trying to push it to the limit is what makes it fun.
Enzo would be a close second. Drives great and does well from 650-800pp. Kind of a tie would be the One-77. With Praiano's tune for it the snap oversteer has been tamed and it's quite quick.
Tied for third (or fourth, I guess) would be the LaFerrari and Huayara. Both are surprisingly planted for the amount of power they can lay down. Only reason I don't have them higher is there's not a lot to do with them given the current content in the game. They can both easily compete against 800pp race cars, but their fuel consumption is so insane that it's just a chore. I like using road cars in the WTC 800, but not when you have to stop for fuel 4-5x per race.
Veneno is a fantastic car. The problem with it is the customization is low and I've not found it really able to do anything that the Aventador twins cannot accomplish, especially the LP750-4 which can have as much (or more) power, is much lighter with all weight reductions, etc and is quicker in lap times in my testing. Great car, I just wouldn't pick it up until you've got the credits to burn. Same applies for the Vulcan, FXX, and Citroen. Lack of customization and/or being a "track" car kind of limits their use though they are all great rides in their own right.
Veyron I'd only buy if you get lucky and snag the stage 5 weight reduction. If we had a proper drag strip and some events for drag racing it would be awesome. Otherwise we don't really have any lakes or rivers suitable for a house boat, lol.
That's just my take based on how much use one would get out of them given the content we currently have.
Edit: ... and as many of each as you want until the invite expires (a month I think).
