Putting my vote(s) in for Rotom-Fan, Starmie, and Lucario.
Let's go for Gravity abuse......
Starmie @ Focus Sash (Center Lead)
Ability: Natural Cure
Nature: Timid
EVs: 252 SpAtk/252 Speed/4 HP
Moveset:
-Gravity
-Hydro Pump
-Blizzard
-Thunder
Starmie is the fastest of all the Gravity abusers. Focus Sash is hold item of choice, in case Scarfers attempt to OHKO.
With Gravity comes an accuracy boost for Starmie's base 120 moves, and can set up for the likes of normally gimmicky Pokes, such as GrassWhistle Whimsicott, and ExtremeSpeed Togekiss (HustleKiss).
(Maybe I can at least get points for creativity here?)
(IMO) The most fallen from grace Poke from Gen 4 to Gen 5......
Rotom-Fan @ Ice Gem (Outside Lead)
Ability: Levitate
Nature: Modest
EVs: 252 HP/ 252 SpAtk/4 Def
Moveset:
-Air Slash
-Hidden Power Ice
-Thunder
-Protect
Rotom-Fan...The forme made for Triples. Since Rotom-Fan hits on the Special side relatively hard, has good bulk, and has a STAB move that can hit non-adjacent Pokes, it can have a great many uses in Triples.
Electric/Flying (iirc) can hit everything bar Ground/Rock typing for at least neutral damage, which HP Ice covers.
Lucario @ Fighting Gem (Outside Lead)
Ability: Inner Focus
Nature: Modest
EVs: 252 SpAtk/252 Speed/4 HP
Moveset:
-Vacuum Wave
-Aura Sphere
-Dark Pulse
-Dragon Pulse/Protect
Most people go Physical with Lucario, but with Luke's move pool, it does great going Special in Triples.
Fighting Gem + Vacuum Wave can put serious dents, or even OHKO's on the likes of Tyranitar, Ferrothorn, and Excadrill before they can do anything that's noteworthy.
Fighting/Dark attacks go unresisted by anything, and Luke's other three moves can all hit non/adjacent Pokes.