Illumise @ Damp/Heat Rock
Prankster
252hp/4def/252spe
Timid Nature (+Spe, -Atk) / Jolly Nature (+Spe, -Sp.A)
-Rain Dance / Sunny Day
-Encore
-Thunder Wave
-Bug Buzz/U-Turn
Role: Support / Weather Setter
What It Does: Leads a weather team and guarantees that weather is set up. As Ilumise has decent enough bulk (better than Volbeat) it can take any priority attack, as it is neutral or resistant to all (apart from Bullet Punch but nobody uses that). Encore forces switches by keeping a 'mon into a move such as Stealth Rock. Thunder Wave cripples sweepers that rely on speed such as Cincinno, whereas an attack stops you from being Taunt bait. I personally use Bug Buzz for power, but U-Turn gives you more momentum.
Good Teammates: Obviously anything that benefits from the weather. In the rain sweepers like Ludicolo, Golduck, Beartic, Relicanth, Emolga, Swanna etc are all good choices, whereas in the sun go for Victreebel, Charizard, Vileplume, Emboar, etc. I don't play Sun so I can't help as much here. If you don't intend to sacrifice Illumise on Turn 1 then Rapid Spin support from the likes of Armaldo (or Torkoal on a sun team) is appreciated to stop her losing 25% of her health by switching in.
What Counters It: While Illumise's bulk is decent it's by no means excellent. Anything carrying a decent super-effective attack can easily OHKO or 2HKO the firefly. Leads like Golem or Cincinno can do the trick, but watch out for Encore or Thunder Wave respectively.
Additional Info: Many prefer Volbeat as it can pass Tail Glow, so keep that in mind as an option too. Personally I prefer Illumise due to her superior bulk (and cuteness)