While its true that NW, CO, and DL give free upgrades, the OP mentioned they will make silver at best. Out of LAX I don't think too many upgrades are going to come their way, especially on the DL or CO transcons.

Since you won't get upgrades on transcons as a silver in any of the skyteam programs the you might go with DL or NW for the fact that they let all elite levels choose exit row seats at booking while CO reserves that privilege for Gold and Platinum elites only.

If you're going to mix your skyteam flying and just credit to one program then I'd also avoid CO. CO has a 50% EQM penalty for cheap tix on NW and DL both. NW and DL reciprocate this same penalty if you fly a cheap ticket on CO if you are crediting to their program. You don't have that penalty if you're using either DL or NW and flying the other, its just with CO.

As for award availability, it will depend on where you want to go with your miles. DL has miserable availability domestically, but pretty decent partner availability internationally. NW is a little weaker with some partners. I don't think CO availability is too bad, though some others may disagree.

I think in your case I might also consider using Flying Blue. As a US resident you'd give up the free upgrades but its one of the few programs that allows you to get award tickets in international first class. There is no mileage penalty on CO or DL. The only fare class that has doesn't earn full miles on NW is V, which is used as one of the cheaper fare classes for international fights. NW K and T classes do get full miles which are usually the cheapest 2 classes both domestically and internationally. AS flights can also be credited to Flying Blue in addition to NW and CO (mentioned above)