Whenever I'm in the mood to play an old school dungeon crawler, I mostly just keep coming back to Four Against Darkness (4AD), because it's one of the best solo engines I've ever come across for that.
It might be easy to overlook 4AD because of how simple it looks. But it's simplicity is actually its strength: Instead of taking an everything-&-the-kitchen-sink approach to solo-gaming (such as with a massive tome of many big tables or etc), it just takes a very minimalist approach. Though table-driven, its tables are small & few. Its focus seems to be more on just making sure that every small bit & piece of it & how it all fits together & works is perfect.
4AD is also so simply organized and modular, it's easy to modify it into whatever you want just by substituting your own tables where they fit into its system. It seems to be so conspicuously designed for that purpose, that it's even hard to resist tinkering with it. In that respect, its versatility & replay-value are practically endless. But I've also got a lot of mileage out of just using the tables it comes with, because they're quite elegantly done & satisfying to play with, too.
Plus 4AD comes with its own RPG system built into it. That system is likewise so simple & elegant, it makes all of the polished work that went into it look easy. But on a closer look, it's actually just one of the most seamlessly streamlined & tight, minimalist versions of old school rule mechanics I've seen. Not only is it conspicuously designed that way to keep 4AD's gameplay fast and fun despite having a solo-dungeon-crawling engine to manage in it, but it also feels faithful to traditional old-school rule mechanics and classes as if we're playing in the 1970's again. It balances those two things perfectly.
The net result is that 4AD is a pure blast to play. The proof was in the pudding as soon as I started playing it: I hadn't had so much fun playing a dungeon-crawler in decades. It just scratched that old school dungeon-crawling itch like nothing else. I couldn't put it down. It became my go-to game for several months.
In fact, I would even consider 4AD to be a good minimalist RPG system in its own right. Except it isn't designed to just run things from other OSR systems unconverted out-of-the-box, the way any other OSR-friendly minimalist RPG systems out there are designed to do. That means you will have to scale-down some of the stats on anything you import from other OSR systems into 4AD's system to make their balance OK, because most things involving stats in 4AD's system tend to be scaled smaller in it. Or considering how simple 4AD's system is to manaage, you could just run a whole other OSR system atop 4AD's solo engine if you like.
Anyway, I just love everything about 4AD. I especially recommend it for anybody into solo dungeon crawls and minimalist hacks of old school rules. But it would also be a great way to introduce younger players to dungeon-crawls, or to play GM-less dungeon-crawls with a group.
[5 of 5 Stars!]