Depends when you have to have your keepers finalized. In my two keeper leagues we get until a week before the season. That way we get to evaluate training camp battles and injuries. Obviously you want to keep guys you think will be gone before you have an opportunity to draft them again. Look at the guys drafting ahead of you and try to predict their needs. Not knowing your league, I would go with Foster. You can never have too much RB depth and it's a lot easier finding a decent WR in a keeper draft than it is a RB.
Keep an eye on the Carolina RB situation. Rumors are that Davis might be done (good for Foster), but the Panthers drafted another RB with their 2nd pick that might push Foster. Supposedly Foster is almost 100% and will be ready for camp. It's his job to lose and when he did play last year, he was awesome.
If Boldin holds out, he's f'd. Green has no pity for whiners, he won't have a chance to get his timing down with Warner and Fitz will push him for the the #1 spot.
Robinson is a waste. I've drafted him in the past based on potential...hoping that "this is the year." He'll drop far enough in your draft you can take a flyer on him in the mid rounds if you think he can get his career on track.
Stokley is a great 3rd reciever, but he's still that...a 3rd reciever. You shouldn't have to use your first pick to nab him, so if you really like him, pick him up later on.