I don't know about all-time, but it's definitely in my top 3 as far as Star Wars goes alongside Empire & ROTJ which are above it simply because the characters and overall story is just timeless and meaningful no matter how many times I see it. Back to Sith, it still holds up. The entire Anakin/Padme relationship is a bit choppy, but I won't deny I felt something for 'em during the final act on Mustafar. For some reason, EVERYONE in that scene just turned it on to a second gear as far as acting went - even Christensen who demonstrated he can act when given the right material. It really sold you on the 'end of an era' theme that just HITS heavy at the heartstrings after knowing Anakin was just an ordinary boy in the first film.
There's also the hinted nuance behind the Jedi and their questionable ways and tactics that adds weight to Anakin's turn and that maybe, just maybe... the Jedi needed to end.

....At least temporarily until they reformed with Luke later proving himself to be a better Jedi than everyone before him in ROTJ.
Not to mention some of the most iconic lines and moments in SW history.
"If so powerful you are, why leave?"
"POWER! UNLIMIIIITED POOOOOOWAH!"
The entire Anakin & Obi-Wan fight
"I have failed you, Anakin. I have failed you."
"YOU WERE THE CHOSEN ONE"
"I HATE YOU"
One of the GOAT Star Wars movies