Gastly/Haunter/Gengar
Tier: A
Availability: Late
Stats: Awesome.
Movepoole: Nice
Altough he is availble just after you beat Aya and you need to have a Linkstone, he is a very nice addition to your team.
With an amazing special attack and a very nice speed he is a very good sweeper.
He has a nice typing which is Ghost/Poison and moves as Shadow Ball and Sludge bomb will do massive damage to your opponent.
If you have for instance trouble with Noel, i recommend using a Gengar.
Since he is immune for normal type moves, he can hit Noel's pokemon hard with a sludgebomb.
I almost want to punt him in Tier S but he also has his disadvantages.
He has a poor defense and hp stat which makes him very vurnable to physical attacks.
Still imo one of the best pokemon you can have in your team.