I would have to say Thomas Robinson as the #1 choice for me right now.
At the moment our only rebounder on the entire team is Baynes - if he were to miss significant time for any reason, then we literally do not have a rebounder on the entire roster. So I feel having a little bit of insurance there would be nice - and Robinson's youth and athleticism should be a nicer fit for the team, as he can at least fit in with an up tempo style of play.
Second choice is probably Bogut. He's a better player then Robinson obviously, but his injury history at his age is beyond concerning. Bogut is one of those guys who you can sign, and if you get half a season out of him you are feeling pretty good. He's on the old side, he's not especially mobile, he's injury prone (understatement), and he's probably pretty rusty given he hasn't really played in some time. Still when he is healthy he can be a game changer defensively, and he definitely fills a need with his rebounding and rim protection.
Next on my list would actually be Gerald Green. I liked what we got from him last year (especially in the playoffs) as a versatile wing who can play and guard multiple positions, knows Brad Steven's system, and can put up points in a huge hurry when he gets hot. Plus he won't complain about his role or playing time, so he won't be a hinderance to the development of our young guys.
Our second unit is very young, especially on the perimeter (Rozier, Smart, Brown, Tatum) so I wouldn't mind a seasoned vet out there to solidify that second unit. I actually would rather have Green back then anybody else based on the player alone - but have chosen Robinson and Bogut above him because I feel that the big / rebounding need is more pressing right now.
I guess the plus side with Bogut is we wouldn't even really need him during the regular season - we could rest him, keep him on minutes restrictions if need be - even if its just 10 minutes a night. If we could preserve him long enough for him to be able to play meaningful minutes in the playoffs, then he could be a legitimate difference maker.
So on that note, here's my perfect scenario. Waive one of the scrubs (probably Nader) and then sign both Bogut and Green. I'd feel much more comfortable about this team (especially come playoff time) knowing we have two additional proven vets on the roster who can make a genuine impact - rather then a bunch of prospects that we have no clue what to expect from.