Can't go wrong with the solo games published from Buttonshy. Sprawlopolis and the other -opolis games are fantastic puzzle games. Rove is another puzzle game that is frequently recommended.

I've personally been playing Maquis, it's a worker placement game that has scaling difficulty. It's also nice for limited table space.

There are a couple of tin box games that I consider to be great solo games. I'm not sure how to classify Doom Machine but the goal is to destroy the Doom Machine before it defeats you. You gain dice to add to your pool when you destroy the components of the machine. It's a pretty simple set up that has high replayability. Tin Helm is another easy to set up game that's more of a push your luck. You defeat monsters and delve into a dungeon in order to collect shards to win the game.

As far as TTRPGs that are solo, I'd recommend Apawthecaria. It's a complete package that all the rules you need to play are in the one book. Put simply, you are a woodland critter gathering reagents to help others in need. Cure ailments and possibly help with issues as you do what you can to gain reputation with the guild. It's a bit on the crunchy side. But thanks to that, it's easy for me to wind down when playing it.