Slowbro - 110 base defence, reliable method of recovery (Slack Off), can learn various moves that will deal with its weaknesses (flamethrower, ice beam), can also learn Calm Mind to boost its special defence and has Regenorator ability.
Umbreon - high def and special defence, reliable methods of recovery (Moonlight/ Wish), only 2 weaknesses, Synchronise ability to damage opponent, Heal Bell for team support, but no viable offensive moves except Foul Play.
Bronzong - levitate and steel/ psychic typing means it only has one weakness, high defence and special defence, slow speed so makes good use of Gyro Ball, access to Stealth Rock and Dual Screens, cannot be poisoned. However, no reliable method of recovery except Rest.
Milotic - mono-water typing means it only has 2 weaknesses, ability Marvel Scale will mean that its defence is boosted when it's either burned, paralysed, asleep or poisoned (you can give it Flame Orb if you want to make sure, along with Aqua Ring and Recover, meaning it won't lose too much health), reliable method of recovery (Recover obv.), has access to Haze which will reset calm mind/ Bulk Up stallers, and Dragon Tail which will force them out.
Reuniclus - slow speed but has enough bulk to use Trick Room, reliable method of recovery (Recover), has Calm Mind to boost sp. defences and sp. attack, has Shadow Ball and Focus Blast to deal with 2 of its 3 weaknesses, has Magic Guard which means it won't be hurt by burn or poison, and will be able to use Life Orb with no recoil damage.
Mew - base 100 in all stats, a huge movepool to choose from, access to Roost, Will-O-Wisp (which will cause an attack drop in the opponent, even with moves like Dragon Dance), along with decent speed.
Cofagrigus - base defence of 145, high special defence of 105 as well, access to Will-O-Wisp to cause attack drop in opponent, access to Haze to reset stallers, no reliable method of recovery but can use Pain Split, access to enough bulk and/ or Trick Room to compensate for low speed.
Porygon2 - not fully evolved so can hold Eviolite, forfeits Leftovers recovery but has Recover, access to a range of attacks that cover most things neutrally, high special attack, Analytic ability which will give it a Life Orb boost for moving slower than its opponent, only one weakness.