Cresselia (aka Lunar Duck) is a decent healer and support Pokemon. 80 HP is good for a basic, +20 Weakness to Psychic hurts it though...no Resistance is common, and 1 Retreat Cost is good. Moon Twinkle makes for a decent stalling attack early in the game if you have to start with Cresselia and you can get a Stadium into play. Lunar Dance is slightly overpriced for 2 Psychic 1 Colorless Energies and only dealing 50, but if you discard 2 Energies attached to Cresselia, you can completely heal one of your benched Pokemon (might be good if Cressy is getting KOed next turn). Like it was mentioned, Cresselia exists to level up into Cresselia Lv X, which'll probably be discussed in a future CoTD.
Modified - 7/10 (Weak to G&G, but a decent tech in some decks with Cressy Lv X)
Limited - 7/10 (Watch out for people using Darkrai #3 from this set and Psychics, otherwise a decent support Pokemon)
Unlimited - 1/10 (Why use it here? Better support options other than Cressy here)