Best upgrades for the 4t65e stock transmission ?

 

The request is too vague to be much help. As it's made from glass, the best mods are those that make it stronger. The best of those are the GMR parts - primarily the input shaft and chain/sprocket set. But since they're not made anymore, they'll probably cost as much as the car is worth now, especially if you found a NIB set.

If you could provide more details on what you're looking to get out of it, we could better point you in the right direction.