I would factor in upgrades using MFU as an option for those that aren't flying enough to reach the highest elite tiers. With that in mind, BA's Premium Economy -> Business upgrade is very well priced. AA's miles+copay option is nice if you tend to buy bottom-barrel economy fares, UA's and NW's "higher fare basis" requirement may not matter if you tend to book on comparatively short notice.

Availability is always an issue, of course.