I'm sure it received his due 5th-generation discussion when the games were new, but it's a serious question. I hardly ever see Uxie on Wi-Fi, and he's apparently been deemed UU by Pokemon players at large, but for the life of me, I can't figure out why. With dual screens and Memento, he has to be one of the best partners for frail set-up sweepers in the game, and I can't think of anything that does its particular job better.
I've been running the following prior to bringing out a Shell Smash/White Herb Cloyster, Swords Dance Scizor, etc, and it's just ridiculous:
252 HP/ 252 Defense / 4 Speed
Psychic, Light Screen, Reflect, Memento
Dual Screen Uxie is nearly guaranteed to take a hit. Playing smart, you'll usually be able to pull off both screens and end with a Memento, leaving room for a game-ending sweeper to run rampant with set-up (especially something already defensive like Cloyster). It'll even buy frailer Pokemon a game-changing turn or too. Worst-case scenario, Uxie can still take at least one hit from almost anything and at least use a relevant screen or Memento. Finally, Psychic, rather than U-Turn, at least packs a moderate offensive punch, for picking off glass cannons or taking a hit and revenge killing.
Yes, it hates Taunt, but so do plenty of other support and utility Pokemon.
So, honestly, what am I missing? Why don't I ever see other Uxies? The only other Pokemon that shares this particularly useful combination of stats and moveset is Cresselia, which trades some of its defense for offense and healing abilities. Is it just too situational? Again, I think it's the definition of "nothing does its job better."