For top-tier elites, those that fly 100,000 miles a year or more, the best benefits are with United and American.
United’s 100,000 mile flyers get (6) international upgrades a year valid on most fares and confirmable at booking (subject to award space availability). In addition there are up to (8) confirmed domestic upgrades a year, and that’s on top of the upgrades earned by lower level elites (4 500-mile upgrades per 10,000 miles flown on United or United Express).
American’s 100,000 mile flyers get (8) international upgrades a year valid on most fares and confirmable at booking (subject to award space availability, but most importantly these are exempt from American’s required ‘buy up’ requirement to pay cash in addition to miles for international upgrades). Unlike at United, domestic upgrades are ‘unlimited’ with no upgrade coupons required.
For award redemption, in my experience, American is the best when it comes to seats on its own flights. For overall award redemption, including partners, American is up there along with United and USAirways (because both are members of the Star Alliance and offer outstanding international redemption options). United does filter out availability from some partners, while in my experience USAirways does not. So while USAirways requires more miles than United for many redemptions (eg 90,000 United miles gets you business class from North America to Asia, on USAirways it’s 120,000 miles) availability is sometimes better with USAirways (just about everything else about them is frustrating, though!).
While Northwest, Continental, and Delta are great for domestic upgrades of elites, which are automatic and unlimited subject to availability. they are all subpar when it comes to award redemption. On the whole award availability lags far beyond United, American, and USAirways. International upgrades are almost nonexistent (except for Northwest elites flying far more miles than required for Platinum). Continental’s on-board domestic product is probably tops in the U.S. (though I’m pretty happy with United) and far above Northwest, Delta, USAirways, and American. So for the purely domestic flyer with 75,000 miles a year or more to earn platinum — and who can book travel at Continental.com so that even discount fares count 100% towards status, there’s a pretty good reason to fly Continental.
Generally speaking, I would only accumulate miles with Northwest, Continental, or Delta if I was flying those carriers regularly and working towards elite status for my domestic upgrades. And even then I would only credit actual flight miles to those programs (and possibly credit card miles using one of the Continental or Delta cards whose spending can contribute to status). I’d credit all other partner activity to another program.
This last point cannot be stressed enough. Who you fly is often dictated by where you live and where you fly. But you don’t have to collect all your miles in one program. Push all your flight miles to a single program if you’re able to make elite status, and earn miles with a single program until you reach your award goals, but then it’s worthwhile to accumulate miles elsewhere so that when it comes time to redeem you have more than one program to choose from. Perhaps United doesn’t have the award seats you want, but American does. That’s a big reason why I’ve never been turned away from an award I really wanted — I have so many programs to choose from, chances are that one of them can help me.