Overview: Tentacool has a massive Special Defense stat that can be abused with Eviolite, making it better at walling special sweepers (eg, Yanmega, Porygon-Z, etc.) than Munchlax. The problem that i usually run into is that Tentacool can't really do anything back because of it's rather small movepool, so it has it's drawbacks.
Type: Water/Poison
It's typing makes Yanmega's Bug Buzz not very effective, making it that much weaker. With the water typing, Special Entei's can't do much with their STAB.
Ability:
Clear Body: (Prevents stats from being lowered by other Pokémon.) Could be better, could be worse. Should probably be used over Liquid Ooze unless Tentacool is on a rain team.
Liquid Ooze: (Damages opponents using leeching moves for as much as they would heal.) Meh.
Rain Dish DW: (Heals for 1/16 max HP after each turn during rain.) Should definitely be used if on a rain team, and could be used if otherwise.
HP: 40
Atk: 40
Def: 35
SpA: 50
SpD: 100
Spe: 70
Movesets:
Supporting Wall
Tentacool @ Eviolite
Clear Body / Rain Dish
Calm
252 HP / 4 SpA / 252 SpD
-Rapid Spin
-Toxic Spikes
-Scald
-Haze/Hidden Power Rock/Sludge Wave
Eviolite along with 252 SpD makes Tentacool's Special Defense 448, which can wall a lot of stuff. Onto the moveset, Tentacool doesn't have a lot of good moves, but you only need about 3. You can throw up Toxic Spikes on the switch, or Rapid Spin if your opponent is using Stealth Rock. Scald is there because you'll need a STAB even though it won't hit for much, and Hidden Power is used to hit Yanmega, though with Haze you can shut down Quiver Dancers. Sludge Wave is there in case you need another (Albeit terrible) STAB. With the ability, you should probably use Clear body just because there isn't a lot you can do with Tentacool's abilities. But you could use Rain Dish if you are putting Tentacool on a rain team.
Anyways, try Tentacool out and tell me what you think of it in this thread.