I really like 700R4 transmissions myself. You have to make sure it is upgraded properly though. Lots of the stock rebuilds don't live long. I use Phoenix Transmission to build mine. They aren't cheap, you're looking at a good bit of money for a rebuild, but they are warrantied for up to 550 horsepower and 3 years. I've never needed the warranty though as they have always worked right. They're the same folks that built all of Boyd Coddington's transmissions. Look for their add in most magazines.
http://www.phoenixtrans.com/html/gmtrans.html