I recommend you choose a water type pokemon as your sixth team member. If you want a pokemon that can take hits choose vaporeon but if you want an attacking water type choose starmir in hg/ss or floatzel in d/p + platinum.
If you don't want a water type choose a great fighting type. Heracross in hg/ss or Machamp or lucario. Don't want a fighting type choose a grass type such as roserade.
Moveset for these pokemon:
Vaporeon: Surf, Blizzard, Shadow Ball, Toxic
Starmie: Surf, Thunderbolt, Psychic, Ice Beam
Floatzel: Surf, Brick Break, Ice Beam, Bulk Up
Heracross: Megahorn, Close Combat, Earthquake, Stone Edge
Machamp: Dynamic Punch, Earthquake, Stone Edge, Bulk Up
Lucario: Close Combat, Dragon Pulse, Flash Cannon, Psychic
Roserade: Sunny Day, SolarBeam, Sludge Bomb, Swords Dance
Overall I'd choose Floatzel because it is a great pokemon that can sweep through a team when it uses bulk up. But it is your choise. I hope this helps