I generally buy NAPA parts. They usually cost about 10% more. I feel they are worth it. I have bought AutoZone parts many years ago. One was a brake master cylinder. It was a new part. So new that some of the port threads were not taped. I tired to tape the parts at work. I returned the part and was given a rebuilt part. I did buy an alternator for a girl I was seeing at the time there. THey put one on the counter, and I looked at it and pointed to a diode or something. They said, that was from the return rack and pulled out another one.
On my MB 240D, I was having problems with the alternator. It had been replaced by the previous owner, with a part from Advanced Auto. I rate their parts better than AutoZone. It was rebuilt in Germany by some firm. I replaced the regulator and it still didn't work. I took it apart and there were some metal pieces floating around in there. The winding wires were broken. I soldier them back and soldier in a resistor that had come loose. It worked but sqeeled. I bought a bearing for it. The wires were broke again. The bearing didn't fit in the housing. It had about 1/8 of clearance. I made a sleeve to fix it. The alternator lasted a few months. Even though it had a lifetime warranty, that was only to the original purchaser, plus you need the reciept.
I replaced it with a higher amp alternator off of E-Bay. That was last week. So it is to soon to say anything about quality. I'd probably go with a local rebuilder. The ones in my area have a good reputation. I would have used one of them, if it weren't for the increased amps I wanted.
Tom