Ubers can be defined as Pokemon with strengths that is way above most Pokemon. They tends to be Legendaries, although not all Legendaries are Ubers.
Wobbufett for example, is Uber despite not being a Legendary because it can lock a Pokemon with Shadow Tag, which affects even Pokemon with Levitate or Flying-Types (unlike Arena Trap ability which does not affect those Pokemon), and coupled with Wobby's Def & SDef, along with Encore & Counter/Mirror Coat, it deserves to be in Uber.
Arceus, Darkrai, all Deoxys forms, Dragon Trio, Garchomp, Weather Trio, Ho-oh, Lugia, Manaphy, Lati twins, Mew, Mewtwo, and Wobbufett all are Ubers.