My choice
Because on this poll I could only choose one level I chose 9th but really it'd be like 9-12. Because, you finally get to match up some feat chains have decent hit points, fight a dragon and survive etc. The 4-8 levels are also fun but I just think that 9-12 is better feat and spell wise. I think you can get into a lot of trouble level 1-3 with even orcs if they swing that crit with the great axe...oops you're gone. The higher levels for me the math gets tiresome and the stacking starts to get absurd along with the save or die aspects.
I do agree that it is the friends and the story that matter the most though. I've had a great time at first level on a one shot deal and had a crappy time on a campaign tha was at 7-12 for the majority so I guess...hell I don't know I picked 9th.
Ramble, ramble, etc., etc., etc..
Would have been easier to anwer in chunks though. low, 1-3, med-low 4-8, med-high 9-12, high 12+ and epic.
Cheers...