Best: Rugged Garen. If you own it, any other answer is wrong. Worst: Phantom Karthus, I think. Maybe Nightmare Cho.
Regret: Probably Shaco. I had some fun using him in 3v3s, but I hate jungling with him with a passion, and his damage output feels pathetic all game long. --- *waits until someone says something funny to quote*
Best: Brolaf Worst: Totemic Maokai (it was on sale along with maokai, so I just picked them both up). Regret: Shyvana. Even though I own every 6300 jungler now, I never play her. I just don't like her. --- Cooler than Freddie Jackson sippin' a milkshake in a snowstorm.
Best: Gentleman Cho'Gath Worst: Renegade Talon Regret: LeBlanc, I stomped pre-30 with her but I'm terrible with her now. --- "They can't backdoor when they're coming through the front door..."~Me Nami is my waifu