Cresselia

Cresselia is the bulkiest Pokemon in DPP OU and with its excellent movepool and immunity to Spikes and Toxic Spikes, it has a place on very defensive stall teams or as a dedicated support slot. Cresselia has a couple of unique advantages for defensive teams: it is notably one of the only counters to mixed Dragon-types such as Dragonite and Flygon in the tier; with Rest, it has a huge quantity of PP and can survive a long time while attempting to freeze the foe with Ice Beam; and Lunar Dance allows it to both heal up and restore PP for Clefable or another valuable Pokemon, which can be back-breaking. However, Cresselia has many exploitable weaknesses. Firstly, the prevalence of sandstorm severely limits its healing capabilities. Secondly, its pure Psychic-typing and poor offensive stats notably gives it a huge weakness to Tyranitar, Jirachi, Scizor, and Heatran, some of the most popular Pokemon in the metagame. Therefore, it requires significantly more team support than most Pokemon to work properly -- on defensive teams, it really wants Heal Bell support, is extremely passive, and often competes for a teamslot with the versatile Latias.