What are the best gaskets to use on aluminum valve covers. I tried rubber and they just became brittle in less than six months. Also what is the best gasket for automatic tranny pan? I don't mind paying the price if they are worth the money.

Cork...for both. 

Thanks Joel! Last year I had the tranny serviced and the kit came with a rubber gasket. The shop owner used a cork gasket instead of the rubber. He said the cork was better but it didn't take long for it to leak.

Cork must be installed perfectly dry. The surfaces of both the pan, and mating surface has to be flat, clean & dry. Over tightening will cause the gasket to break.The quality of the gasket is important, too. There are some really THIN cork gaskets out there, for both valve covers and engine/trans pans.
First, be sure the gasket is actually leaking. Trans leaks can be hard to find the origins of, since the fluid will wick/seep everywhere.

On the valve covers I used fel-pro standard thickness and the thicker gaskets. The thicker gasket sealed better but a year later they started to leak to. Might be over tighten. The tranny pan gasket it definitely the cause of the leak. I think what happened was the cork gasket was old. My friend that has the tranny shop has retired five years ago. He rents out two bays and keeps one for himself to do small jobs. In the previous post I mentioned he used a cork gasket instread of the rubber one in the kit. I know he used a cork gasket on the shelf rather than a new one!