Mario Power Tennis > Open
But get the GCN version, the Wii version has forced motion controls that I didn't like.
The best I can say about Open is that, unlike Ultra Smash, it was actually finished. You can't disable chance shots or anything. Whereas when doing exhibition matches in Power Tennis you can turn off Power Shots and it's pretty much just a refined 64. Even with Power Shots it's still more refined than Open. Open's worth buying if you can get it for like ten bucks but I wouldn't pay more than that now that's the online's inactive.