Pink Floyd. I still think The Final Cut was a good album, and they had more left in them.
Soundgarden came back, but I wish they hadn't broken up to begin with. I think King Animal was a great album... it just came 14 years too late.
The Mars Volta fizzled when they kicked Jon Theodore out (or he quit, whatever happened). That band had so much going for them.
Mr. Bungle broke up too early. I liked the direction they were going with California and that band, in all, was so amazingly talented and quirky. Always a surprise listening to them.
Hendrix and Buckley, I agree. Sketches for My Sweetheart the Drunk is awesome, and I really wonder what it would have been if he were alive to realize it. He had limitless potential. Such a great vocalist and also a great rhythm guitarist and songwriter.
The Beatles. I am not the biggest Nirvana fan but I thought they were going in the right direction. I thought they were going to get better.
I think Rage had already peaked when they broke up--they were never able to evolve.
AC/DC with Bon Scott was the real AC/DC to me... Brian Johnson never really compared.
Alice in Chains had effectively broken up in the late '90's with Layne Staley not really being around to contribute anymore. The last few tracks they did (Get Born Again and Died) were awesome. That iteration of the band had so much left to offer if they could have ever gotten healthy.