I'll only talk about Star Alliance here, since this is the Star Alliance forum...
If you are going to buy C or F fares, bmi's Diamond Club is a slam-dunk choice. Huge bonuses for premium fares, plus the best redemption options in the alliance make it the best choice. If you're buying Coach, United's Mileage Plus is a good option because of the Systemwide upgrades (SWUs) you'll earn when you hit the 100K EQM mark. You'll only be able to upgrade three of your round-trips per year, but it's better than nothing.