DM Yourself by Tom Scutt is my favorite solo RPG product. It's the one that finally cracked the code for me on how to play D&D adventures alone. Every other solo engine, I feel, is focused on generating content to play through, which is not an issue I had as a solo gamer. The AD&D 1e Dungeon Masters Guide contains so much content for generating an adventure that a lot of the systems of other solo engines felt redundant to me, and most of them boiled down to just another set of random tables. DM Yourself finally addresses the problem of running a character. It does this through a number of innovations centered around basic choices you would make as a player, and gives you a framework for how to use them. It is written for use with premade adventure modules for D&D 5th edition, but can be adapted seamlessly to randomly generated content and OSR play.
The most important innovation of DM Yourself is the method for handling "Default Behaviors" and "Time Consuming Actions". In OSR games every action has a time cost associated with it, and DM Yourself advises to choose 3 of them prior to visiting any new location. This sets up a "Binding Decision" for your character that you must follow through with, even if you find out that such an action would cause them harm. There is a neat little card in the back of the book that sets up Binding Decisions, such that you can keep track of your choices when taking new actions, and can turn it to different choices as you progress.
The second innovation that improved my solo play was the suggestion to play for 1 main character and 1 sidekick. Using henchmen is a fundamental part of OSR play but as a solo gamer I found it laborious to juggle a full complement of henchmen, and I didn't really see the difference between a henchman and a full player character in a solitaire game. Stripping the number down to a single character and henchman allowed me to immerse myself more fully in the role of my character, while also having the extra support for combat and other challenges.
The immersion table was a nice addition and I used it more than I thought it would, and the rest of the advice in the book is fairly helpful for structuring solo play. The Character Sheet add-on is very useful for keeping the systems straight, as are the quick reference appendices.
There is a page near the end of the book for adapting DM Yourself to OSR games, and it covers mostly all the minor tweaks needed. Most of the book's additions to solo 5e are part of OSR games anyway, such as determining encounter distances and logging time. There are a few things I would make a note of though, such as mapping.
Mapping is a big part of the player's experience in OSR games, but for the solo gamer this will basically have to be spoiled. Every TSR module comes with a map of the dungeon, and it's better to just have the full dungeon map in front of you as you play and move your characters by grid spaces per turn, and only read room descriptions after having entered the respective room. This way you don't really need to worry about the difference between "skim reading and deep reading", but you lose the risk of getting lost in the unknown. Although this isn't an issue if you're randomly generating the dungeon, such as out of the AD&D DMG. 5th edition D&D adventures also come with plentiful gridded dungeon maps, and their online publications have "player" versions with hidden information removed, so the maps themselves are technically better for solitaire dungeon crawls, but the problem is that 5e itself doesn't have exact rules for dungeon exploration.
As I mentioned in an earlier post, TSR D&D and its OSR derivatives are very simulationist games where every action is expected to occur as if it would in the real world. For that reason I feel that narrative structures like the Mythic GM Emulator are a bad fit for D&D as they assume a generally incompatible style of play. Tom Scutt's DM Yourself is meant to work explicitly with D&D and can fit both modern and old school adventure modules, and randomly generated dungeon and wilderness adventures.