There are several Pokemon that could fill that niche, SimplyG. Rotom-W has access to Will-o-Wisp, Thunder Wave and Toxic as well as a STAB Volt Switch, and it also has solid bulk and a great typing, giving it multiple opportunities to switch in against Pokemon such as Tornadus-T and Heatran and start spreading status. With only a single uncommon weakness, it's very easy to fit onto a team.
Mew has access to a relaible way of inflicting every status bar freeze, and also both U-turn and Volt-Switch in its vast movepool. With Taunt, Roost and a ton of different attacking moves, it can be tailored for almost any role, and it's 100/100/100 bulk is excellent too. However, it does suffer from having a lackluster Psychic typing, which can leave it vulnerable to popular attackers.
Amoongus can learn Spore, the only 100% accurate sleep move in the game, which gives it an excellent niche. It can also use Stun Spore to paralyze Ground-types, something few other viable Pokemon can boast. Though it doesn't learn U-turn or Volt-Switch, it does have Regenerator, which allows it to pull off a "hit 'n' run" style set well. It also has great defenses and a decent defensive typing, with a crucial water resistance that's oh-so-useful in the current metagame.
From Hell's Heart I Stab at Thee