I guess it depends on if you are talking max distance or just simply Favorite driver.
Dont know if I have one for distance seeing as I found I can get a Cheetah or Leopard out almost as far as a Sidewinder when hyzer-flipping any one of them for distance.
As for just favorite driver, I guess it depends on the course Im playing. Generally, I love the Teebirds for simply being so reliably straight, so would pick that any other time but lately I am finding I love Cheetahs which are very underrated and can use some plastice choices, and it makes me want to give a Gazelle a shot because I think its probably in the same boat.
Leopards are also amazing discs and discounted a lot as being for beginers, but I've been getting it and the Cheetah out to about 350' at times and that happens to be where I can get a Sidewinder or a P PD out to, so I've been wondering, "Whats the point?" I havent tried the PD since getting them out that far so, who knows, but had the Sidewinder out.
Then another disc I am going to try some more today, seeing the distance I was getting with a Leo, is the River. Its still very new so I need a lot more time with it but it seems like its going to be a very good disc for a very long time.
All things asside and just in the moment, I will say Cheetah.