S rank
I would say Uxie has enough unique and important niches in the current meta to warrant this prestigious position. It has a set for just about any team archetype out there and no other mon can really pull them off as well as Uxie does. Some notable sets include
- Anti-lead Uxie with Imprison (a little gimmick, but a relevant one nonetheless)
- Physically defensive Rocks Uxie (one of the few rockers in the tier that can handle many different types of offensive pressure at once to lay down rocks)
- Bulky CM (does it differently than Mushy as it's waaay faster)
- Weather lead with Memento (most weather teams default instantly to Uxie for its ability to set up weather, rocks, and keep up momentum with Memento. Nearly unrivaled in its ability to do this.)
- Fast Dual Screens support (best screener in the tier due to Memento being able to keep momentum)
Outside of its great support versatility, it's one of the bulkiest mons in the entire tier as it cannot be OHKO'd outside of boosting moves and crits.
252 SpA Choice Specs Typhlosion Eruption (150 BP) vs. 252 HP / 0 SpD Uxie: 259-306 (73.1 - 86.4%) -- guaranteed 2HKO
252 Atk Black Glasses Liepard Knock Off vs. 252 HP / 0 Def Uxie: 156-186 (44 - 52.5%) -- 18% chance to 2HKO
252 Atk Life Orb Sneasel Knock Off vs. 252 HP / 0 Def Uxie: 179-213 (50.5 - 60.1%) -- guaranteed 2HKO
252+ SpA Choice Specs Rotom Shadow Ball vs. 252 HP / 0 SpD Uxie: 276-326 (77.9 - 92%) -- guaranteed 2HKO
This thing eats hits all day. Not to mention that's all without any defensive investment. The best part of Uxie is that there is no best defensive spread for it, you can just tailor it to take whatever hit you need it to on your team. Not only that, but it sports one of the most impressive support movepools in the tier (U-turn, Knock Off, Stealth Rock, Yawn, Thunder Wave, Dual Screens, Memento, Rain Dance, Sunny Day, Trick, and Imprison) and a pretty nice offensive movepool so you can pick what you want and don't want to wall your sub cm set (stuff like Giga Drain, Dazzling Gleam, and T-bolt).
Overall, Uxie's a fantastic mon in the meta right now and, in my opinion, is a clear candidate for S rank.