My A6 Quattro is sluggish when starting. AZ & O'Reilly say my battery is good and its most likely the starter or alternator but my alternator gauge shows a steady charge. What do you experts think?

 

Get your hands on a good multimeter and check the voltage at the battery when the car is off and then again when it is on. You're looking for about 12.2/12.4 while it's off and about 14.2/14.4 when on.

I honestly would not trust the big box stores to test batteries.

Doing the above test will give you a good idea as to the health of the alternator.

Aside from that more detail will help. Does it sound like the engine is turning over multiple times before it starts? Does it sound like the starter is slow/weak?