Descent
I finally decided to give DXX Rebirth a try, rather than fooling around with CPU cycles etc. in DOSBOX. WOW. This is a great source port, up there with Quakespasm (for Quake) and Yamagi (Quake 2). Like with those source ports, you can really customize how closely you want to game to mirror the original experience (I prefer non-filtered textures for games from this era, as filtering them just makes them look too blurry).
Playing this game with a flight stick and keyboard is simply amazing. One of the most amazing gaming experiences I've ever had; it is worth getting a decent flight stick solely to play Descent and Descent II (and Overload for that matter), though of course once you've got one you've got dozens of amazing flight sims stretching back 30 years or more waiting for you.
Gabriel Knight (original DOS version of course)
Never had this back in the day, though I had a friend who had it and I watched him play it for a bit.
I have recently been taking a break from RPGs to delve into adventure games, as I never really got into them as a kid (except for Kyrandia 1, QFG series, and Leisure Suit Larry). I started with Secret of Monkey Island, which I finished for the first time but had mixed feelings about (in summary, some of the best EGA graphics of all time, amazing MT-32 music, truly LOL writing, awesome insult swordfighting puzzle, but some annoying stuff where I got stuck mostly due to the interface [the semi-hidden hatch on the boat, not realizing you could scroll on the map for Monkey Island]). I gave the sequel a try and liked it more (especially that dynamic iMUSE music), but started to get annoyed with comedy adventure game logic and wanted to try something else.
Now this is more like it. It's a Sierra game so I'm sure I'll run into some bullshit at some point, but I was able to get to Day 2 without getting stumped, because stuff just made sense. I love how the game gives you a bunch of locations to explore right off the bat, even though some of the stuff is optional. Giving you access to optional locations in a game is a huge deal and I think not enough developers realize this. As long as you aren't wasting my time too much, give me some nice locations to look around in, maybe give me some optional story details... it goes a long way toward making the world feel like a world, rather than just a series of locations you have to visit in order to progress the plot (this is what soured me on Unavowed, btw, as I felt I was just being ushered from screen to screen following the story, without any chance to explore on my own and just get an idea of the world the game takes place in).
Anyway, some of the greatest graphics in an adventure game, a truly great soundtrack (that sounds godly on Sound Canvas hardware), interesting characters, and fun locations to explore. What's not to love about Gabriel Knight?