I actually "like" (as in, they're better than others) Delta.

When I fly Delta, by and large they're pleasant, on-time and the planes are clean.

On the other hand, United are rarely on time, their planes very nearly always filthy and their staff is, at best, curmudgeonly. American are, by and large, about on par with United, except that they're typically more money, a fact only offset (slightly) by cleaner aircraft.

I know everyone waxes poetic about Southwest, but, in my experience, their fares are generally on par with (if not higher than) their competitors. Although their staff are pretty good and their aircraft clean, their routing options can be ridiculous. Unless I'm playing the mileage game for status upgrades, a 2 or 3 stop routing is never as good as a one stop or nonstop.

US Airways is hit or miss, depending on whether you're getting a former America West crew and/or agent or an "old US Airways" crew/agent. Basically, the AW folks are good, the old US folks frighteningly bad.

All told, just my 2¢. I tend to think that everyone's least favorite airline is the one they just flew and since Delta's biggest, they're the whipping boy for the industry.