Best in the series for me were 1 and 3 (1 until the Trigens show up at least)
Also Blood Dragon rocks. Just finishing New Dawn now. Constant horrible rap music the enemies play does my head in but the expeditions are good fun, especially the downed ISS one which has some awesome sound effects. Its a bit more like an RPG than the others, you really need to level your character up to tackle harder enemies, everything seems to kill you instantly at first.
Also really enjoyed 5. Didn't think I would as it was getting a bit samey by 4. Thought the story and the setting were good though and the fishing was great.
Just hope they reign in the animal attack nonsense in the next one though. People getting mauled to death by rampaging skunks and turkeys is a bit much.