Hahahaha.
I played an AAR over at The Other Site that I enjoyed a lot, but never finished. I've played the living crap out of all of these, back in the late 80s/early 90s and even recently.
I've never played any of those, though i've heard of them of course. If I were to choose just one to start which should it be?
This.
I'd start with Pool of Radiance.
Pool of Radiance was the first ever Gold Box D&D game, I think. There were two others released after this one, as continuations of Pool of Radiance - Curse of the Azure Bonds, and Secret of the Silver Blades. You could continue characters from PoR to CotAB, and then to SotSB. This was basic D&D ('basic' as in not many frills to it) back in the day. If you start here with PoR, I highly suggest you check and be sure about level caps. If you go with a demihuman, they're probably going to have limited growth. Humans, of course, don't have level caps. But it's your game - you can always replace a character in a later game, if you wish. You don't have to import characters into sequels.
PoR's GUI is pretty primitive and will take some getting used to (hell, all of them are like that, to be honest); I only tolerate it well from the fact that some of my muscle memory from navigating the menus remains. However, later versions/sequels improve upon this.
The Krynn series included, to start with, Champions of Krynn. It was followed by Death Knights of Krynn, and finally, Dark Queen of Krynn. If you prefer DragonLance stuff, you might go with this first.
Pools of Darkness was another D&D Gold Box game, which I believe was stand-alone, but you could import characters I think from Secret of the Silver Blades.
There's also Gateway to the Savage Frontier, which was followed by Treasures of the Savage Frontier.
There was also a Dark Sun version, which I had/have, but it didn't do a lot for me so I never dived too deeply into it. It was more arcadey and that didn't appeal at all, IIRC.
Lots of goodness, here. Depending on the price, this will be an insta-buy for me.