I'd have to say on a level of technical proficiency, Dream Theater or Liquid Tension Experiment.
In terms of Old School, one band that revolutionized the music industry that helped to inspire a lot of modern rock and metal would be Judas Priest. Hell, Judas Priest was the first rock band/metal to utilize two guitarists. Not to mention, Robert Halford technically has a much wider vocal range than Michael Jackson was ever capable of. Hell, it's also been said that Judas Priest made the first thrash song "Dissident Aggressor", which Slayer covered out of honor.
Of course, with all the different genres, it would take a while to list all the bands I find amazing and explain why. So those are just two examples from different eras and different genres.