I've been spending some time listening to, researching and playing solo rpgs.
So here are some initial recommendations for RPGs
A game designed with solo play in mind is Ironsworn.. And also 1-1 or with a Gamemaster.
The pdf version is also free.
The characters make vows which drive the quests and there is a setting called the Ironlands .
There is an example of play at a podcast
Knave and Maze Rats
A classless system using osr (old school rules) ,game principles, Knave mixes in modern ideas, which fits on 7 pages. It's a brutal system for an older style of play.
It's sister system is Maze Rats which comes in at 13 pages and is a more complete system.
It's super simple and small enough I've got a printed copy to take with me on travels.
Dungeons and Dragons 5E
It's not a perfect game for solo rpgs as it's designed for 3-5 players. But it's one that many people know how to play, which makes it appealing. Plus there is a tonne of support for it in adventures, options and online tools.
You can manage a whole party yourself or use the concept of Side-Kicks introduced in Tasha's Cauldron of Everything.
The Wretched is a solo journaling RPG which uses a jenga tower as a tension building tool.
Each day you're making journal entries as you try to cope as the last survivor on an intergalactic salvage ship. It's basically replicating the Alien feel.
I heard about it on the Die Alone Podcast which reviews a couple of rpgs in the same group.
Let me know if you've got other solo recommendations!