Personality wise my favorite has gotta be Michael. For once in GTA I enjoyed playing as someone older, with more experience. Plus, his views on modern culture and his actions towards his family very much reflect my own, so there's that.
Trevor comes in close second. Although I didn't like him at first
, but he did grow on me over time. He's the sort of character GTA has always needed, since there's precious few stuff you can do with him that seem out of character. Both killing and helping random bystanders make sense due to his constant mood swings.
Franklin on the other hand poses no competition towards the other too. I found him absolutely bland and boring. Plus I just can't relate with the whole "hood" culture. I find it ridiculously stupid. Franklin did earn some points in his favor by trying to get away from his background, but it still pops up in every single interaction with Lamar (whom I despised for his, as Franklin put it, tiny motherf*ckin' brain), and he stayed a boring character for me throughout.