Speaking as someone with a hefty seven-figure total mileage balance (which doesn’t compare to some folks I know with an eight-figure balance), my own preferences are as follows, in order:
Starwood: hotel rooms are almost always available, plus points transfer into most airline programs is at 1:1 — plus 5k bonus for transferring 20k. Starwood Amex is the best points-earning card to the extent that spending earns 1.25 miles per dollar on all spending when transferring points in 20k blocks to airlines with 1:1 ratio, which is better than the AA Mastercard for instance. Not to mention the card is cheaper.
American: I’ve never had any problems with availability. Partner awards are great. All miles earned count towards lifetime elite (we’ll see if that feature lasts).
United: Availability on UA metal ain’t what it used to be, but Star Alliance provides for amazing redemption options. Required mileage is low by most standards (e.g. 90k miles for a business class ticket to Australia from the U.S.). No fees for last minute bookings.
Alaska: Provides great availability. Only 10k miles to confirm an upgrade from any fare. That would get you from Boston to Juneau. Good partnerships (AA, BA, CX, QF, KL, CO, NW, DL for example). 20k domestic awards on Alaska metal.
Delta: Have had good experiences with availability. Good partnerships. But this is bottom on my list because I simply don’t trust the program, award costs have risen, and I can always transfer points in from Membership Rewards so why go out of my way to earn directly from Delta?
(Oh, by the way, I’m not including Diners Club Club Rewards or American Express Membership Rewards on this list, perhaps I should and need to rethink my ordering. I’d probably rank Diners second, Membership Rewards behind American.)
I stay away from:
USAirways: I burned my miles there. What have I really lost by playing it safe and claiming awards? I’ve gotten use out of the miles, which is all I’d have gotten later anyway. And since they participate in Membership Rewards I can always deposit more in my Dividend Miles account if I need to. Current balance: 236
Continental: Continental availability is terrible. The only saving grace is that they have partners who offer better availability, like Delta and Alaska. Mileage prices are on the high side for valuable awards. Also a Membership Rewards participant, so why would I want to earn exclusively in the Continental program? Exclusive earning, for me, goes to programs whose points are inherently valuable and where I have a more difficult time accumulating. Current balance: 719
Northwest: Love the elite program for domestic-only flyers, hate the redemption program. Massive price increases for awards. Saturday stays required for domestic standard awards. And I’ve found availability on NW metal to be pretty poor, at least as a non-elite there. If I ever want to claim an award on Northwest metal I’ll transfer points to Continental or Delta from Amex, or I’ll use Delta or Alaska miles I’ve chosen to build up. I use these miles when I can, keeping balance low. Current balance: 2719.
America West: They just lack the route network and partnerships to offer valuable awards. And while they offer some attractive fares, especially for last minute travel and for their premium cabin, I don’t really want to fly the airline anyway if I can avoid it. Current balance: 60.
I don’t meaningfully participate in the Southwest, Airtran, or JetBlue programs either; short expiration of credits and inability to redeem for the kind of awards that I value.
I have some British Airways miles but don’t go out of my way to accumulate. I’m glad that I don’t hold a big balance with Qantas, what with the gutting of their award chart.
I have plenty of points with Amtrak, Hilton, Priority Club, and a few others. I consider those somewhere in the middle of the pack. Amtrak transfers 1:1 into Continental and Midwest (but they dropped bigtime in my book by pulling the rug out from under United transfers). Priority Club redemption is excellent in my limited experience and points are easy to accumulate. Hilton redemption is less than stellar.