My personal favorite is the Ubermacht Zion (BMW M6.) Although I do have an undying love for the Futo GT. My favorite car in real life is the Koenigsegg Agera R, but Rockstar got everything wrong with its in game counterpart (Entity XF) that Koenigsegg got right with the Agera.
It could change though, GTA's car spawning system has been flawed and around since GTA III and there's probably tons of cars we haven't seen yet and won't see until GTA Online drops.
Honestly though, my favorite car in any GTA Game was the Turismo from San Andreas (Ferrari F40).
Also, I have a theory about the collisions in game..
I think there's a random death system with car collisions. Sometimes I'll hit a car head on at top speed and suffer minor injuries, while sometimes my character will fly out of the windshield, dying without ever hitting anything from a low speed collision. Same thing with running into rocks, sometimes my car will just explode from something minor.
On a side note, I think a ton of features are being withheld until GTA Online or the next gen/PC launch. I don't know where $265 million dollars went, but what we have right now doesn't seem like a quarter billion dollar game. I think GTA Online is gonna be a gamechanger, kinda like what World of Warcraft was to the MMO scene. I also think when the game drops on next gen/PC it will blow everyone away.