It depends on what kind of tranny do you want and how much you're willing to spend.

The OEM transmissions were:
Auto 727
Manual 904

If you want to spend a little more their are aftermarkets for both types depending if you want a 5 speed manual or an overdrive auto.

I like to have a manual transmission on my car, so I would go with either A904 4speed, or the Keisler 5 speed.