Over a page into it and no one yet has listed The Fish that Saved Pittsburg?!? Talk about star power- Dr J and Kareem in their prime, along with a ton of other NBA/ABA players. Stockard Channing, Jonathan Winters, and a few other late '70s known actors.
Seriously, only watched it as a kid because a huge Dr J and Sixers fan. Movie is just okay, but better than some of those mentioned already.
And I don't understand anyone who doesn't have Hoosiers in their top 3. For me, top 3 movie of all time, not just sports. Plot wasn't just about basketball, great music, awesome cinematography, and some great acting- hence the Oscar for Hopper. The scene Hackman tells Jimmy he doesn't care if he plays and he finally misses a shot is epic direction.
Another basketball movie no one has mentioned- Heaven is a Playground.
Finally, may not be a true "basketball movie", but I would add Finding Forrester to a must-see list.