So let's give a hand (or, tentacle, rather) for . . . Octillery!
Octillery has an amazing movepool to splash around in (no pun intended) as well as two great abilities, but is held back from stardom by its horrible Speed. However, due to Suction Cups, it cannot be switched out by Roar or Whirlwind, making it a thorn that will not go away. This, combined with its abilities, makes it a topnotch Underused Pokemon.
One of the best Octillery movesets that exploits its strengths is this:
Octillery @ Life Orb
20 HP / 252 Atk / 236 Spe
~ Energy Ball
~ Ice Beam
Besides the fact that this offers amazing type coverage, this is a good receiver of stat boosts, since nobody can move Octillery once it's sent it. The Speed values can get pretty high if an Agility is added, and with Special Attack passed to it, it can sweep all over with its broom of doom, provided no good special walls are in play. In Underused this can be a real team annihilator. Just add water.
Ballom's Rating: 4/5
While I've always had a soft spot in my heart for Octillery (aside from its abominable sound), his lack of good Speed and relatively low stats take it down a notch.