1. Niko Bellic. I liked the whole immigrant dynamic that was unique to the character. A character with a dreadful past who comes to the land of opportunity to escape it, only to dive back into a whole other world of violence and potentially lose his loved ones. Technically all by is own hand in seeking revenge. Niko suffers one the potential trope in Rockstar protagonists in that they're the overly sarcastic, reluctant criminal who thinks they're surrounded by idiots and knows better than everyone else, but there is at least more to him.
2. Trevor is of course the best protagonist of GTA V, but there are times where it gets a little too over the top for my tastes. It certainly helps him fight through the dissonance that has existed in the relationship between player and protagonist for so long in Rockstar games. In this regard, he is perhaps one of the most interesting and unique protagonists. He is second to Niko for me.
3. Johnny is cool, and its interesting to see the his established relationships within the gang develop and unfold after Billy returns to bring unease into the gang once again, but he isn't too dissimilar to Niko and his trope.
4. Michael can be an entertaining protagonist because he is this new role of washed up veteran in a mid-life crisis family part of his character, but I feel he quickly becomes overshadowed by Trevor. And his mid-life crisis bits in the story were some of the weakest parts of the story.
5. Franklin is probably the least interesting, being the same played out reluctant criminal role without the witty sarcasm and backstory of Niko or Johnny. At most, he is likable because he is a reminder of classic San Andreas that had a better gangbanger protagonist. Lamar kind of carries his story arc because Lamar is the greatest.
6. I don't remember too much about Luis to be honest. He was a hired tough guy for Gay Tony who had most of the personality throughout the story. Not too different from Franklin with having a side character carry him.