I held status with the following airlines: (past, present)

AA Exec Plat (present)
AA Plat, Gold, no status
UA 1K (100K), 1P (50K), none.
US Chairman's, Gold, Silver (pre merger)

I like AA Gold for the low (25K) level. US call centers, free bags, better seats, etc. For me, having a USA call center is HUGE. Even with no status, AA treated me decently. When I had to call for a simple thing with UA, I usually had to deal with India or Manila. Not the same as a USA call center.

DL silver may be OK but I would think with so many ways to get silver w/o flying (credit cards--Silver status in the past for transferring 50K from AMEX MR to DL), I would guess that upgrades would be more difficult than with AA.

My vote goes for AA. One way awards, reasonable saver award inventory on awards, etc. Upgrade (500 milers) don't presently expire. Of course, don't expect to get upgraded too easily on a peak hub route (recent example: LAX-DFW this Friday morning). I am lifetime Gold (1 Million Miler).