It depends on what you're looking for, and varies by location.
Most of the general parts like fluids, wires, screws, etc... I go to Advance. They're fairly knowledgeable there and have decent selection. Not the best, but usually good enough.
The O'Reileys around here is all young guys who don't know a drain plug from a spark plug, so I tend to avoid there.
Autozone seldom has what I need, and lots of their parts are junk. I've had some problems with wheel bearings and strut mounts from there that didn't last too long. Not to mention the one around here always has long lines, and seems to be the only parts store in town that is willing to cater to Spanish speakers, so it's packed and no one speaks English.
Napa isn't bad. Fairly knowledgeable, a little pricey, but they can usually get their hands on stuff the others can't. Plus they usually have plenty of diesel items in stock and on sale.
For the real parts I usually order elsewhere. Tousley Ford has all the OEM parts for prices you can't beat. I also tend to get most of my Dodge/Fleetguard parts from Geno's Garage.
My local NAPA has excellent customer service and I go to it over AutoZone. They go the extra mile. The prices are usually competitive although not always the lowest, but I still go there first and only go to AutoZone on the rare occasion that NAPA doesn't have the part.
I like the alternative recommendations... I checked out rockauto... good prices for sure... I'll tell you one thing... I wasted some money at Advanced the other night on new lightbulbs for my car... wow do they have a mark-up on them... I think in the future it'll just be rockauto for me...
Haven't set foot in an Autozone since they canned an employee for using a firearm to protect the life of his supervisor. Will never go there again. No big loss, their parts are usually sub par. I go with Orelly, the local guys are helpful, or Napa, for the same reason. If I'm buying electric parts, always OE or Napa.
eta: my local advance is always understaffed, so they're my last choice. Again, autozone isn't even on the list.
I always go to O'Reileys they usually have the best prices and they can get AC Delco Parts and the Warehouse is just 5 mins away. So if i need a special part i can get it quick without waiting days on shipping like the other Auto stores...
Napa is good but usually over priced
Autozone stop going there bc of their electrical parts suck.
Advance because its the close. Never O'Reiley, there place always looks like a waste land in the summer, never paying attention to the lawn. Makes the neighborhood look bad. Kind of reminds me of that home in the hood that has cars on blocks and you would never know anyone lived there.
I've bought things at all of them over the years, but NAPA is the go-to during the week, but I can also put it on the farm account and then pay it back.
I spent plenty of time at a independently owned shop. If we needed an intake gasket for a gm product we went with Oreillys. Anything and everything else was napa. My brother owns a motorcycle shop and sends all his head work to a local napa. My vote goes to napa.
I do rock auto if i can wait, then advance or oreilly. Haven't been to a napa in a while, but never an autozone, their stuff is crap.
Basically, i like to have options when I purchase parts. The three I frequent usually give me several options in manufacturers with various quality levels (like brake parts) to choose from. Autozone give you duralast, which, in my opinion, is typically crap. Napa closes too early for me. I usually buy my parts in the evening or weekends, when they are closed
I feel as if auto zone's parts are of the poorest of quality of any part store listed on the poll. I usually go to advance with good results, and napa is a bit more expensive.