Cresselia

Cresselia is a physical tank who excels at preventing physical threats, most prominently Groudon, Rayquaza, and Garchomp, from annihilating one's team. Unlike Lugia, Cresselia lacks a Rock-type weakness, meaning she takes less damage from Stealth Rock and Stone Edge, while sporting roughly the same physical durability. Due to the lack of Stealth Rock weakness, she can reliably defeat Dragon Dance Rayquaza, unlike Lugia, making her the ultimate physical wall. Cresselia also makes a great dual screener with only three weaknesses, titanic defenses, a passable Speed stat--with a Timid nature, she is capable of outrunning the many base 90 Speed Ubers that do not run a Speed-boosting nature--and perhaps most importantly, Lunar Dance, which enables a sweeper to come back into play at full health.

However, Lugia competes with Cresselia, due to her limited movepool, overall inferior stats, and the need for sunlight support in order to have reliable recovery in the form of Moonlight (with less PP too). As a dual screen user, she also faces significant competition, though Lunar Dance makes her unique in her role. Nevertheless, these are only limiting factors. Cresselia's movepool is more than enough to adequately support her teammates, her stats enable her to carry out her purpose, and Groudon's constant presence on the battlefield makes Moonlight more reliable in Ubers than in the sandstorm-dominated OU metagame.