I think it's a little crazy to pick a rookie that we haven't seen play. He looks great in college and on paper, but may be a complete bust in the NFL. I don't think that will happen, but to pick him over a proven feature back seems like a wee bit of a biased answer.
That said, you have the long term consistant impressive output of Curtis Martin, the power that Dillon brings to the table, and McGahee's all around potential. Course, one could say that Martin is old (though we've said that for years and it hasn't stopped him) Dillon is one dimensional, and McGahee is injury prone. Of them, I'd probably take Dillon.
Let's wait for at least a carry or two before we annoint Brown as the best back, ok? He might well be there, but a little patience would be prudent.