It might help to know what type of experience you are looking for?
Scarlet Heroes excels at the core fantasy elements (dungeon crawling, wilderness exploration) but also has an interesting urban adventure mini-game.
Ironsworn/Starforged are curious games, extremely well-written but without prior experience with fiction-first play (Blades in the Dark, Powered by the Apocalypse, Fate) I would hesitate to recommend to someone experienced with RPGs but new to solo. (Where as if you have never played RPG before and are starting out solo, I think it is an excellent starting point.)
Zozer's SOLO & HOSTILE: SOLO take the core solitaire experience already built into Traveller and expand it to troupe play. HOSTILE: SOLO is a complete game in a single book where as the original SOLO requires some prior knowledge of Traveller solitaire play.
Two Hour Wargames' RPG title (5150 New Beginnings, Larger than Life, All Things Zombie) do an impressive job of procedurally generating just about everything you might need. Unfortunately the rules are a bit unusual compared to other RPGs and may require multiple readings/actual play to understand. (With that said, these games have produce my most successful and enjoyable solitaire play.)
The all new Mythic Gamemaster Emulator Second Edition is an amazing book with many, many, many examples covering the entirety of topics one might need to consider in GM-less play but it is not necessarily a game. It could be... but you can also pair it with an RPG of your choice. The original Mythic ("Blue Book") excelled at Sandbox adventures but was a bit iffy on others. The 2nd Edition does a much better job of letting you dial between full on Sandbox and Quest-driven adventure.
This is just like... a sampling out of titles. The Drifter/StarDrifter, d100 Dungeon/d100 Space, Calpyso, NoteQuest, Four Against Darkness, Thousand Year Vampire, The Wretched, and 100s of other titles are out there but they all cater to different genres and styles of play (as well games that more or less can be played GM-less out of the box like Freeform Universal.)
It is hard for me to recommend _just one_ without any knowledge. If you are holding your hand shaped like a finger gun to my head ... then I'll say this: If you wanted to read about and understand what GM-less play _means_ from a philosophical level than I'd say the Mythic Gamemaser Emulator 2nd Edition or the Solo Gamemaster's Guide (Modiphius Press) might be your best bet. If you wanted a complete game that is free and of a very high quality, Ironsworn is hard to beat (though it has some nuance and you'll read conflicting accounts of how to play it. I will strongly suggest that contrary to a lot of actual plays, Ironsworn is _not_ a "solo journaling" game. It is a fairly gamist game with a quite a bit of mechanical widgets to interact with.)