Viton and Buna are basically going to be your main options. There are some more exotic materials but unless you want a custom shot of 20,000 and 6 month leadtime it's probably going to not provide noticeable improvement.
If you REALLY want to get into things, I suggest checking out the good old Parker 5712 catalog
If you don't design hydraulic parts for a living , you just want a simple answer. The simple answer is both are ATF safe. Viton is better for high temp applications, Buna is better for low temp. I would go Buna. Viton is good if you have a hydraulic system that gets very hot and is not expected to operate at low temps. Buna is better for standard operating temps and is seen to seal better at extreme low temps.
If it was under hood where it is exposed to 180-220F+ regularly, I might bias towards Viton or HNBR. At work we usually say Buna is good to ~160F before it will start to degrade at long life (2000-5000hr type thing) Viton is solid up to 200F at long life. HNBR even above that.
But as it's going to be in a car cabin where it will see -20F storage to 120F operating, Buna is right in it's sweet spot. If it stores under pressure or if you operate at -20F (which you won't, it's a vert top) you MIGHT see some weaping with Viton as it doesn't perform at very low temps as well as Buna.
This is a case where the cheap option (Buna) is the best option.