It's very hard to choose favourite characters, Aoyama-Sensei is really good when it comes to writing lovable characters!
But here's a list of Characters I love the most (Hopefully it won't be too long):
1) Ran. It physically pains me that I'm not Ran
She made a better person and affected me more than any fictional (or even REAL) character ever! After reading DC she became my role model! She's very nice and considerate. She's strong both mentally and physically (I WISH I had her karate skills). She has a childhood friend that is a love-interest (I really wish i had one). She always says the right thing. She treats people very nicely. And the reason can go on and on...
2) Sera! SHE KNOWS JEET KUNE DO! She's a detective! She's so cheerful! She's Akai's sister! She's so mysterious! I can't hardly wait to figure everything about her, and i REALLY hope after knowing everything about her, Aoyama-Sensei won't remove her from the series
3) AKAAAIII!! Shuichi is the best and i don't even think i need to list the reason we ALL love him!
4) Kaito Kid
6) The detective boys (specially Mitsuhiko and except for Genta)
12) MAKOTO <3
as I said the list is too long!
As for the characters i dislike: Genta and Yusaku are it!