Let me first start by saying, my voting comes with as little as possible bias. So it will come as no surprise here that I don't rank the Cardinals number one in the deepest RB core. Let's take a look....
The top is headlined with two guys with injury concerns and rookies who will make their fair share of mistakes. It's hard to say this is not a talented unit, but there are just to many questions surround the top two guys. Powell and Smith, barring injury ahead of them, will find a tough road to the final roster.
Probably the top two guys in the NFC West. Turbin last year showed that once Lynch cannot carry the load anymore, he can fill in easily. Lynch to me is scarier than Frank Gore, because he isn't a one trick pony. He can break tackles, he can elude you, he can punch you in the gut, and he can outrun you. This isn't to say Gore isn't a monster. If he gets in the open field you cannot bring the man down. (i.e., the Cardinals)
This unit is filled with potential. Pead, who split time with Jackson a lot last year, is a solid back. Will he replace the great Steven Jackson? No. But he can be a really good starter in the NFL. The rest of the guys on the roster have potential, no doubt, that's why they're in the NFL. Some won't make it, but Stevens, and Richardson are guys I would keep and eye on.
Gore is still a premier back in the NFL, James is a monster, Hunter and Dixon are solid backs. Those guys will be on the roster this season. Lattimore will find his way to the IR, which is expected, and Hampton and Harper can either be put on the PS or be cut. To me the Niners have the best RB unit in the NFC West, let alone the NFL.
So, the Cardinals were number one in quarterback depth, and I will rank them as third in running back depth, due only to the fact that Mendenhall has more experience than the guys on the St. Louis roster.