I think one of my favorites was definitely the buggy as a more extravagant vehicle. Other than that I would love the Infernus, I loved that car in all GTA games and its just badass especially with mods that made it into a Lamborghini proper.
Totally the helicopter! I'd use that cheat to go up into the air and just snatch the controls from the pilot lol! Really nice controls for the vehicle surprisingly. Bullet was also very good as it would rank as my favorite land vehicle, but overall, I would say that the helicopter really was the bomb!
I liked Sultan and the monster truck. Sultan was a great car and the handling was pretty good, at least for me. And, the monster truck was very fun to drive. I liked the Turismo as well.
I love motorcycles in real life so my favorite will be the Sanchez and also the BMX. I'm a bad driver whenever I used 4 wheels in the game so I always try snatch a Sanchez or a BMX. I always store one on my garage by the way.
Probably Sultan, because of the tuning you can do with it, like adding spoilers, bumpers, there were so many options to choose from. Oh and I liked Inferno too, that thing was fastest.