You know whats weird? I've played most of those, and i remember alot of them. Yet, I constantly mix up their titles.
I'd suggest playing them in chronological order with their releases so the systems get better cus i always hate playing a sequel and going back to a previous game and having things be clunkier.
Do you like soul crushing difficulty? Pen and Paper? How about having to install fan-mods so the game doesn't crash constantly? Then boy, do I have a game for you: TEMPLE OF ELEMENTAL EVIL DAWG.
Not really. Baldur's Gate is a classic and if you use Baldur's Gate TuTu or Baldur's Gate Trilogy to mod it up to the standards of its sequel you'll probably have a good time, though Baldur's Gate II overshadows it in a lot of ways. Planescape is also a safe option since it's not really a game so much as it is an interactive novel with crummy combat sequences. Icewind Dale and its sequel are very combat heavy and demand more strategic thinking from the player, though I'll disagree with Dave Snider by saying that the first IWD is better than the second. Neverwinter Nights is boring as death if you play it alone, and if you wait 30 minutes or so I'll probably have my NWN2 blog up for your perusal. Can't say anything about Dragonshard or Demon Stone, other than that they're not especially well-regarded.
Baldure's Gate with the trilogy mod.
While I love the Icewind Dale games, those games are more combat focused and their story is kind of bland.
But if you're feeling like a beat-em-up with your D&D, hop on Steam and buy the "Chronicles of Mystara" bundle. If Final Fight made love to an original D&D rulebook, this would be their child. IMO, the best classic beat-em-up of the genre (or at least tied with River City Ransom for the honor).
Go for Baldur's Gate. I just picked up the exact same bundle having never played any of these games before (the closest I ever got was Dragon Age: Origins).
I'm probably a good 30-40 hours into BG and only on Chapter 4. It's already one of the best gaming experiences I've ever had.
I followed this mod/fix guide, which contains only the most essential mods. I wanted to experience BG1 as close to vanilla as possible but still have things like bug fixes and widescreen support. I figure that on my subsequent playthroughs I will use the BGT or TuTu stuff, but I kind of wanted to go in as fresh as reasonably possible.
Don't play the first BG at all, its boring. A D&D game that starts you at level 1 is boring. Read a synopsis and play 2 if you want. Or play PST because its awesome. My favorite game, in fact. IDW2 is also good but doesn't have a story to drive it along, so you need to just want to play it for the gameplay (which is quite good).
I'd go for Baldur's Gate. Planescape is one of my favourite games of all time and I think Baldur's Gate 2 is a more compelling story experience than BG1 by modern standards, but since you're asking this question it appears that you're interested in the D&D setting and rules as much as you are interested in the games themselves. With that in mind, I think Baldur's Gate (fully enhanced with BG Tutu mod, some fixpacks and a widescreen mod) is the best out of any of these games for introducing you to the fundamentals of the world, without compromising on depth and non-linearity.
Planescape's combat is inferior to BG and its class system is only a really simplified version of what you usually find in D&D. Baldur's Gate (the first) gives you a breadth of free choice over who to be, where to go and what to use. While playing it you can look forward to the greater polish of the sequel, with its superior storytelling and dialogue, although Baldur's Gate 2 compromises on some of the non-linear world exploration aspects in comparison to its predecessor.
Icewind Dale would be a bad choice. I can't speak for the second game, but I've tried to get into IWD1 time and time again and failed to make much progress. It's just an unforgiving combat slog, minus many of the more interesting overarching story elements. In all probability any one of BG1, 2 or PS:T will kick your arse at low levels since they don't make allowances that modern gamers are used to. IWD assumes you're cool with this and does it harder.
I haven't played ToEE or NWN, but I hear parts of NWN2: Mask of the Betrayer are up there with Planescape in terms of writing.