Of course, humble opinion, always:
- Guitar Hero 2 (you didn't need special controller)
I'm going to disagree here. I'm a huge fan of the instrumental rhythm games. For context, I own every Guitar Hero ever released for Xbox 360 minus Guitar Hero Live which I own for Xbox One, as well as owning Rock Band 3 and Rock Band 4. Full disclaimer: I think overall the Guitar Hero series is fantastic - I even thoroughly enjoyed the final game in the original run, "Warriors of Rock" for its song selection, DLC system, and support for as many players to play whatever instruments they desire. I would say that, honestly, Rock Band 4 with the Rivals expansion might be the best game to grace the genre. It has the simultaneous online multiplayer (which should have been included in launch, but I digress), an excellent song selection with great DLC options that expand every week, along with support for Pro Drums which are by far the best way to play the game. It also has the added benefit of significantly improved charts for songs with better HOPO detection (it was unnecessarily strict in Guitar Hero 2 which showcased skill, but at the same time made the overall game tougher for the moderate players - this was fixed in Guitar Hero 3 and at the latest Rock Band 2), more interesting chart movements in the note placements (they were fairly barebones in Guitar Hero 2) and the option to expand your library significantly for relatively cheap. $2 per song which has an easy/medium/hard/expert Bass, Guitar, Vocal, and Drum chart means that the amount of content in Rock Band 4 is not only out-of-the-box greater, but has more variety.
Now I will agree that Guitar Hero 2 had a one-of-a-kind setlist, but that's also tough to beat when it was a game that came out when most people that are fans of the genre were younger and had a more malleable music taste. But Rock Band 4 also gives me the option of pursuing whatever genre I like, which also benefits it over Guitar Hero 2, which was mostly classic rock to alternative rock and metal. Rock Band 4 has a wider genre selection. It's honestly a shame that Rock Band 4 never came to PC. If the Series X has backwards compatibility with Rock Band 4 (which it should), it might actually convince me to get the Xbox someday even though I don't really want one. While you need the actual instrument controllers, that's what really makes the genre shine in my opinion.