I started watching this show early last year and finished earlier this year, and I'd highly recommend it. It's one of, if not the best, written shows I've ever seen. The writing, characters, character development, cinematography, drama, action, and humor are all amazing.
I enjoy all of the main cast, so I can't really choose a favorite, but here's my rankings of the seasons:
1. Season 2
2. Season 5b
3. Season 1
4. Season 3
5. Season 5a
6. Season 4