I think saying parker is the best back on the team is being a bit closed-minded. Parker is a really good back, and a really, really great value considering how much production we've gotten from him for the payout.
but lets not overlook the fact that parker has not, nor will he ever be what we call a "complete back". meaning he's got superior speed and work ethic, and a constantly improving ability to cut and break tackles. BUT, he's not got the ability to run between the tackles consistently and force defenses to overload the middle to stop him.
both russell and especially mendenhall have that ability, along with good to great speed to break it long.
some here are really high on russell, but I'm not holding my breath. he needs to get on the field and produce before I annoint him heir to the black and gold rushing attack. Mendy on the other hand is going to be something special. he has the rare blend of size, speed, power, and vision to supplant parker as the starter from day one. I don't see that happening for a littney of reasons, but it's possible.
as coach, I'd really work russell out during the preseason, make him carry the load for two of the games and give him 20+ reps each... see if he's got it. I'd sprinkle in mendy in those games and give him a half or two in the other games just to get some time in before he rips up the AFC north.