Yes, Alaska! Alaska partners with both NW and American, and if you're based in SFO there's more places you can go on Alaska once you stop flying to MSP on NWA. Also, keep in mind that Alaska is more likely (tho you never know for sure!) to stay in existence on its own the next few years than either NWA or Delta. (It's not so likely you'd lose the miles from either of the latter, but you may get them transferred to someone less useful.)

Keep in mind that when you give a partner's FF number when booking a flight (in the case either Delta's or Alaska's), you may or may not be eligible for exit rows and/or upgrades (depending on the specific relationship between the two partners). If this matters to you, look closer into each set of reliationships. If you're happy with just choosing normal seats, and just want to accumulate miles in some place you can actually use them later, then you have more choices open.

Btw, you can't transfer miles out of NWA to anywhere, so if you choose to earn NWA make sure you can use them as such. (You can redeem NWA miles for Alaska flights, tho.) I don't think you can transfer Alaska miles either, but in that case you're more likely to have use for them in SFO. I don't know whether you can transfer out of Delta or not...