I'd like to break it down by player a bit first:

Grant- Went from no string to first string last season. Hey, it happens that way sometimes. Gotta love a guy who can come in from the deep bench and surprise everyone and produce! I think it will be a little different this season without Favre and defenses will test Rogers to the ultimate limit. He might be a one year wonder, and if you've watched enough football, you'll know what I'm talkin about.
Mendenhall- Sure, the guy's definitely got some skills. BUT, he's still a Rook and competing for time with FWP. So, "maybe" in the future, but don't see jack coming out of this season. Oh, and right out of the blocks he's had hammy issues. So, take that for what it's worth….aint even gotten to full contact camp yet.
Stewart- I really like this guy….ALOT! He's had his fair share of injuries in college, but hey, there's been others who've been able to have great NFL careers. I like his situation in Carolina as well. He should produce nicely THIS season as well.
McFadden- Dunno about this guy. Sure, he's got the blazing speed and all that. BUT, I don't think he's gonna be able to live up to the hype that's been surrounding him this season. The Raiders line aint all that great. He's not even close to Adrian Peterson, and he DOES have an O-Line. Mostly a rookie for a QB to boot. Don't see Oakland getting better on that side of the ball in the near future either.
Larry Johnson- Hey, the guy WAS as STUD at one point…two years removed now. I feel his better days are far behind him. Sure, he's got some "fresh" legs as far as NFL RB standards are concerned, but the 2006 season definitely took its toll on him. Last year he was a broken man. I don't think he's got much left in the tank after all the beatings he's taken any longer. KC's line, a mediocre QB, and a lack of receivers make it even rougher for him.
Michael the Burner Turner- Yeah, we have all loved watching him come in for LT and lighting it up! That dude's got MEGA skills! BUT, he's in Atlanta….but wait! Not an entirely bad scenario if you ask me. He's definitely gonna be the feature back with zero competition. He's got the ability to take it to the house on any play. Only real problem is a lack of a passing game and terrible defense. If he can be a gamer, like I think he can, that will help out on both sides of the ball.

So, here's who I'd choose first to last:

Turner
Grant
Stewart
McFadden
LJ
Mendenhall

There you have it, my opinion. Not only for the here and now, but for your future "franchise" team. :)