Delta is the probably the best among the domestics. I use them twice a year, and its typically hassle free and comfortable enough for a domestic coach seat. Jet Blue is the only other one I can think of that provides an arguably better experience, but they don't have the routes I use so I've only flew them once. I have minimal experience with Alaska, but they seem good.
United is absolutely god awful. I've had a bad experience with them nearly every time. Southwest is fine if you can stay on top of the booking system, but if you slack you get squashed in the middle seat between the biggest people on the plane. Fine for short routes.
Keep in mind, you can use a United or American card to book with the airline partners. I would never use United miles to fly United, but Singapore or Copa? Yes please
The problem with all of the perk systems is that the airlines are constantly adding rules and finding ways to make the perks less valuable. I use a United credit card to accrue miles, because they seem like the easiest to redeem for good routes. AA miles often require sh!t routes and extra stopovers to redeem. Delta is the scroogiest for mileage redemption. The last few years I've moved towards pointed based cards that give you a statement credit. That way you book the flight you want and get a rebate for it. I like this approach, because then you can still collect miles for the flight itself, which you don't collect when you redeem miles. Unfortunately, airlines are now cracking down on collecting miles from partner airlines (such as collecting United miles while flying on Copa) unless you buy one of the more expensive fare classes, so you are only guaranteed to collect with the actual airline itself. Fookin silly, but what are you going to do about it
The best value with miles is redeeming them for business class flights, although you are essentially giving up a trip for free to have more comfort.
Ultimately, you're best of collecting miles with whatever airline you use the most.