The characters I really like are pretty much the popular ones. But then right now I gotta say I'm really liking Hidemi Hondou aka Mizunashi Rena/Kir. She's so underrated and is seriously too nice for the B.O. not to find she's just not one of them. I love her voice and how she looks and the fact that she's a CIA undercover and that she puts up that evil sneer in front of Gin and all of the other BO members but turns completely different behind them. I always look forward to episodes she's in.
I like Vermouth too, she's a complex character and one sly woman but to be honest though she owes a lot to Ran and Shinichi, Shinichi, Ran and Ai has got lots to thank her too. It's good to see that a villian like her actually keeps a sense of indebtness or gratitude, enough to keep Conan and Ai secrets unknown to the organization and sometimes to the point of keeping them alive in times of danger.
I love Akemi and Akai too, they have such an interesting love story though short lived and tragic.