For classic fantasy dungeon-crawling fun times, you really can't go wrong with the one that started it all: Dungeons & Dragons.
Though that doesn't really answer the question, because when it comes to D&D, the question on everyone's mind is "which edition?"
My personal favorite is AD&D 2nd edition, most likely because of my Infinity Engine bias. The Baldur's Gate saga, Icewind Dale, and Planescape: Torment all used 2nd or "2.5" Ed AD&D, and I used to play it a lot as a teenager with friends when I used to DM. It's an older, rather strange ruleset, with lots of eccentric rules that probably wouldn't make much sense to a newcomer (or to anyone, for that matter). It's sort of the weird uncle of the D&D editions. And it has a bunch of great, since discontinued campaign settings like Planescape, Dark Sun, and Ravenloft. (And some less-than-great ones, too.)
I also like the 3rd edition, which is more of the "mainstream" edition among players (or at least used to be). It's a lot slicker, streamlined, and generally tends to make more sense. The trade-off is it has less "personality" than 2nd Ed. Still, it's well thought out, fairly complex but intuitive, and it has depth. It was used in some video games like Neverwinter Nights and Icewind Dale II, and maybe some others I'm forgetting. A good ruleset overall.
There is also 3.5, which is a rebalancing of 3.0. Don't know much about it, to be honest. I think NWN2 as well as some other video games in the early 2000s used it.
4th edition brought a lot more streamlining, balancing, and simplification. I don't much care for it, because IMO it changed some fundamentals that made it no longer feel like D&D. It's too "gamey" for me, with added emphasis on combat and less on role-playing. It also messed a lot with the Forgotten Realms setting, advancing the timeline by over 100 years, and bringing a bunch of weird changes that fans mostly hated.
Don't know much about 5th edition, as I lost interest well before it was released. I also haven't played the original AD&D, or 1st edition D&D, which is more of a board game, as far as I can tell. Perhaps someone else can give their opinions about those.