Can't say anything about Prestige since i couldn't watch it so far, but i only read good things about it.
Gladiator without a doubt is a good movies (Great score, good actors (Joaquin Phoenix and Russell Crowe), interesting story and great settings), but there are some things that prevent it from being near the top of my list:
a) Too many historical inaccuracies
b) It wasn't really thought-provoking in my opinion and that's what a movies has to have in my opinion to be considered one of the best ever, otherwise it is just "plain" entertainment
I could watch Requiem for a Dream, Beasts of the Southern Wild, American History X, 2001: A Space Odyssey, Breaking the Waves, Brokeback Mountain, Cloud Atlas, Magnolia, Memento, Lost in Translation, Her, Seven, American Beauty, A Serious Man, Blade Runner: 2049, Dogville, Life is Beautiful, The Seventh Seal, Inception, Moonlight, Manchester by the sea, Matrix, Nocturnal Animals, The Machinist or Shutter Island multiple times and always discover some new "angle", but that is not really possible with Gladiator in my opinion.
If it's "only" about entertainment, then i would rather watch movies like City of Good, The Godfather, Scarface, Goodfellas, The Revenant, The Green Mile, Pulp Fiction, The Big Lebowski, Schindler's List, The Pianist, Forrest Gump, Rain Man, Kill Bill: Volume 1, Hacksaw Ridge, Inglourious Basterds, Fight Club, Once Upon a Time in America, Deadpool, Departed, The Usual Suspects, Saving Private Ryan, all of the Lord of the Rings movies, some of the Star Wars movies, Apocalypse Now, Batman Begins or The Dark Knight.