Larry,
It really depends on how much money you would like to spend. I think that the best trampolines are the rectangle ones, but they can be really expensive. Check out the trampolines they are using on the media section of this website.
http://gurutrampboards.com/ You can search for them on google as “gymnast trampolines.”
I personally recommend the 14’ or 15’ trampoline you get at your local Wal-Mart or Sam’s Club. They usually cost about $200, and often go on sale. The 14’ provides a harder bounce, but is a smaller surface area if you fall. The 15’ is a softer bounce, but you can still get as high, and do all of the same tricks. It just takes a little bit more effort.