Cobalion without a doubt.
Has been UU since its inception. That remained relevant and competent for 4 consecutive gens, consistently surviving the power creep while doing the same roles very reliably. Pivoting, setting up rocks, checking extremely important mons throughout plenty different metagames. Cant help but respect it, it is just a very complete and useful mon. Probably at its worst nowadays, but hardly useless.
Id say this mon perfectly defines UU to me.
2nd is definitely Slowking. IMO, the history of UU as a whole is tied to this mon very tightly. This mon isnt number 1 mainly cause it hasnt been as consistent as Cobalion in my eyes, but lets see: Gen 2 relevant, Gen 3 very relevant, Gen 4 it was NU but very useful by UU standards. Gen 5 also RU but very useful as well, similar venue in Gen 6. Gen 7 is where it vanished mainly cause Slowbro is UU, but gen8 and gen9 brought it back to relevancy thanks to teleport / chilly reception. Excellent defensive glue that has found its uses as a pivot or wincon very consistently.
3rd is probably both Nidos, I think Nidoking and Nidoqueen should share the same place, they have gone through similar roads, just that the bulkier profile of Nidoqueen has been more preferable throughout the gens. Sheer force revived these mons, making them very scary breakers with pretty attractive defensive and offensive profiles.
4th is probably Scizor in my eyes. Dont think anything has had such an oppressive grip in the tier as Scizor did during Gen 7 UU. Absolutely vicious. Has remained relevant on the 3 gens with biggest power creep (now OU but it will very likely return before the gen ends). Just for its reign of terror Id say it is one of the mons to (dis)grace UU ever
5th is Crobat 100%. Tornadus-Therian has no voice in this discussion, it is a mindless pivot but it is only UU cause of the insane power creep. Crobat has been doing the bulky pivot role for generations. At its best in current ORAS UU imo, solid in gen 5, definitely at its worst on Gen 7 but Id say bullying the omnipresent amoonguss has its weight, then heavy duty boots made it plenty relevant on good teams in gen 8. Good mon and very useful altogether.