I hope this is not a loaded question but I would like to know which flight sim website store is considered the most reliable, or maybe the top 3.  I ask this only because not all add-ons will be in the in-game marketplace and will need to be purchased elsewhere.  I had 5 or 6 places I used to purchase add-ons for FSX and prior as well as X-Plane but it is a small nightmare to manage all the various software keys and emails for re-installs, especially a year or more later after purchase.  Anyone have a good central spot for purchases?  I'm US based but not sure if that matters.

These tand to be the places I would look at. You probably know all of these, but either way, you can trust all of these places.

Occasionally a bit limited in scope, but Just Flight is a good place to buy from since it has very reliable servers, patch/update deployment is good, and it has a genuinely usable loyalty bonus scheme which does actually grant you pretty good discounts off further purchases from there.

PC Aviator is another one which is reliable and often has quite good sales on products fairly regularly.

Not always the cheapest but possibly the most comprehensive and with a reliable server, is Simmarket. It tends to get lots of the latest products fast, so even if you don't buy from there, it is a good place to browse stuff.

Flight1 is worth a look, particularly since it is well known for not quibbling over offering refunds if you are unhappy with your purchase.

Simshack is another good place which is reliable and offers plenty of choice.

Something of a rival to Avsim forum-wise, Flightsim.com nevertheless has a good online store which is worth checking out.