Hard choice, really. I like most of the gym leaders in Sinnoh (except Byron, he's just got no emotion)...my top picks are either Candice, Crasher Wake or Fantina...I'll just say all of them ^_^
They all have totally different personalities, Candice being the high spirited, cute girl who's a master of the sub-zero Ice-type. Crasher Wake is out there, a far different contrast from the other Sinnoh leaders (maybe even ALL the gym leaders in the history of Pokemon), his attire of choice, going for the Mexican wrestler look..his Pokemon of choice are a nice mix too, a mix between fast sweeper, setup and wall, he knows his competitive sets.
Then, there's Fantina, who's the flashy gym leader and master of the tricky Ghost-type. She's fantastic, always wearing her designer dress, having the iconic French accent of hers, and also being a Pokemon contest fanatic, that's what sets her away from the others. Imo, the Sinnoh gym leaders all have their positives, and are a delightful bunch, but the winner in the end is Candice. She just picks up your spirits as you battle her, she's like Whitney, except without the devil spawn I like to call Miltank.