These are all the Pokemon that get the ability prankster:
Banette-Mega, Cottonee, Illumise, Klefki, Liepard, Meowstic, Murkrow, Purrloin, Riolu, Sableye, Thundurus, Tornadus, Volbeat, Whimsicott.
The most used ones are Klefki(although not so much), Sableye and Whimsicott, Thundurus can also make goos use of it but by asking what is the best prankster user I assume you didn't mean to set up Nasty plot's and such.
Your best bet would in my opinion be Sableye. Here are some good sets for Sableye:
100% Priority
Sableye @ Leftovers
Trait: Prankster
EVs: 252 HP / 4 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Recover
- Taunt
- Will-O-Wisp
- Toxic
Mean look
Sableye @ Leftovers
Trait: Prankster
EVs: 252 SDef / 252 HP / 4 Def
Calm Nature
- Mean Look
- Toxic
- Will-O-Wisp
- Recover
Trolleye
Sableye @ Leftovers
Trait: Prankster
EVs: 252 HP / 4 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Recover
- Substitute
- Will-O-Wisp
- Confuse Ray
Trickeye
Sableye @ Iron Ball
Trait: Prankster
EVs: 252 HP / 4 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Recover
- Trick
- Will-O-Wisp
- Taunt
I've used all of these sets before and they work very, very well if used correctly, that's why in my opinion Sableye still out-classes Klefki.