I'm perpetually experimenting with different solo-engines, random tables, oracles, geomorphs, etc. But whenever I want to take a break from that & simply have some fun playing an old school dungeon crawler, I keep coming back to Four Against Darkness (4AD). Because it's one of the best solo engines I've come across for that.
It might be easy to overlook 4AD as a good system for solo dungeon crawls because of how simple it looks. But its simplicity is its strength: Instead of taking an everything-and-the-kitchen-sink approach to table-driven solo-gaming (such as with a massive tome of tables or etc), it just takes a very minimalist approach. Though table-driven, its focus seems to be more on just making sure every bit and piece of what it does 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's clearly designed for that purpose, & hard to resist tinkering with its tables. In that respect, its versatility & replay-value are practically endless.
Plus it comes with its own RPG system built into it. Which likewise looks so simple, a child could play it. But on closer look, it's also one of the most insightfully streamlined & elegant interpretations of old school rule mechanics I've seen. Not only does it keep all of the gameplay in 4AD fast and fun despite having a solo-dungeon-crawling engine to manage in it at the same time. But it also manages to feel 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 4AD is a pure blast to play. The proof was in the pudding as soon as I started playing it: I haven't had this much fun playing a dungeon-crawler in decades. It just scratches that old school dungeon-crawling itch like nothing else. I couldn't put it down. It became my go-to game for several months.
I especially recommend 4AD for anybody into solo dungeon crawls and minimalist hacks of old school rules. But it could also just as easily be used as a great way to introduce younger players to dungeon-crawls, or to play GM-less dungeon-crawls with a group.
[5 of 5 Stars!]