5 was my favorite until I played 4.
Let me say that 5 is still my second favorite and it's really close but 4 just does everything in 5 better. I'm playing the series out of order but I think that has helped keep things fresh. I started with 5 and New Dawn then moved on to 3 and Blood Dragon. I'm playing though 4 now and it's just so good. There's so much variety in mission types/activities/weapons/animals. The karma events make the world feel alive without being annoying. I felt like in 5 you couldn't go 2 minutes without running into resistance activities and if you didn't want to trigger the forced story missions early you had to ignore them. I'm also really enjoying the art direction. The graphics really hold up even against 5 and ND. The collectables are interesting and even the vehicle activities are more complex and engaging. Looking back 5 seems like a simplified version of 4 and I don't know if they had complaints about 4 being too bloated but I'm sad they cut so much out. I haven't done much of the storyline yet but I'm already loving Pagan Min and can see why so many people adore his character. 5 still holds a special place in my heart, especially the characters, but I'm glad I didn't start with 4 because I might have been a little disappointed playing the others after.