Just to be contrary, I like the O/U, for the following reasons:
1) It is VERY clear when the action is open.
2) Allows for much wider variety in shell combinations.
3) Can easily and quickly check bore for obstruction.
4) Keeps spent shells off the ground (good for reloading).
5) Aesthetically pleasing.
6) Incredibly simple to take down and clean.
7) When hunting, you can have two different chokes. I often use I/C and I/M and select the barrel depending upon how far away the dog is ranging.
8) To unload or "make safe" takes about 1 second.
That being said, I tend to like different actions for different purposes. When breaking fast-moving orange disks, I'll always go for the O/U. I have recently started hunting with an 870, but have shot hundreds of birds with my autoloaders and O/U as well.
IMO, they're all good! The only thing I wouldn't want to do with an O/U is HD, but that's just personal preference. Some people actually like coach guns for that.
For all-around use (or if God forbid I could only have one shotgun), I'd probably pick a 12ga pump, with a 12ga autoloader following closely behind.