The legendary R&B singer took the stage at Allegiant Stadium Sunday for the Super Bowl Halftime Show -- and he put on a medley for the ages ... going through all his biggest hits, including 'Caught Up,' U Don't Have to Call,' 'Superstar' ... and eventually, 'My Boo.'

Alicia Keys came out and started singing 'If I Ain't Got You' on the piano ... and then transitioned into her duet with Usher, and it certainly stood out as a major moment.

Jermaine Dupri also came out and helped Usher sing 'Confessions' -- and that transitioned into 'Nice & Slow,' and then 'Let It Burn' right after that. And finally, 'U Got It Bad.' It was during this part that Usher went shirtless and was joined by H.E.R. who did a guitar solo.

Usher came back out and started skating onstage alongside Will.I.Am ... and they did their hit 'OMG' -- with a bunch of other backup dancers/skates who were spinning circles around 'em.

Lil Jon was the next big guest ... and this led into perhaps Usher's biggest hit, 'Yeah,' He put on an incredible dance number. Luda eventually came out as well and rapped his verse too.

It was a great show, obviously -- but it was surprising that Usher didn't bring out Bieber, as many expected him to be there ... y'know since he was literally there in Vegas this weekend. Plus, as we reported -- Usher and JB had been in direct talks in recent weeks about this.

Guess the dude wasn't up for it after all .. in any case, it was a halftime show for the ages!