Overall, hard to say I like each one for a certain reason. I love how much each one varies. If I had to choose (and thus, from favorite first):
John Stewart - He is the one I can most identify with and the one who seems the least static in personality and so forth, despite his efforts to be static. He has undergone more change than the other ones and has grown the most, in a manner that was sensical on a comic label that is not known for growth and development. Also, I love his tendency towards approaching things in a calm manner.
Kyle Rayner - I love the fact that he makes with his ring the very things I would think of. It's akin to having your very own comic book character in many ways.
Guy Gardner - Brash as hell and so damn obnoxious its hilarious. I personally think he and Stewart are the best lanterns. Guy is one person I can also identify with. Having have his origin, he and Stewart were my first exposure to Green Lantern.
Isamot Kol and Vath Sarn - Their interactions are wonderful. Individually they are fun as well.
Dkrtzy RRR and Mogo - I've always been a fan of these lanterns because of how creative they were. One is a biosentient math equation that can erase people from existence by entering their minds and another a sentient planet. Each was my first exposure to such things and changed the way I view comic book characters and concepts in general. I also blame them for making me like Grant Morrison and his wacky concepts despite not being made by him.
Kilowog - I've always seen him as Thing with a green lantern ring. His plain spoken nature, strong and studry self encasing an intelligent and capable mind alwasy seemed cool to me. And poozers.
Hal Jordan - Yeah, Hal was never really my favorite. I could list a good number of lanterns before him beyond what I have already posted. I get that he is reckless and so forth but at the same time, so is Guy Gardner and he seems to pull it off better and it seems more ingrained with his personality in a more natural manner than Hal. Hal always seemed like....he had no real or true personality and when he did, he came across as irksome. I don't know. I don't hate him, but he always seemed like a second tier character to me.