I love games that play well solo, as well as games that are small enough to take places with me.
People have already mentioned Friday, One Deck Dungeon, and Onirim, but I would definitely echo those people.
Others I genuinely enjoy:
Almost any of the Tiny Epic series of games works well solo. (I haven’t given Western or Quest a try solo, but Galaxies, Defenders, and Kingdoms all work well alone, especially if you can pick up Ultra Tiny Epic Kingdoms, a board game the size of a deck of cards.)
Deep Space D-6 is easily a top five game I’ve kickstarted. It’s tense, fun, and tough to win. I know they’ve gone back for a second printing of the game with a small expansion that should be shipping soon.
Star Realms’ single player is really fun, too. You do have to buy a card pack to get the single-player opponents, but the whole game is cheap enough that an extra $5 to get the single player and co-op variants are worth it. (Ascension is a little more expensive, much bigger box version of the same game play, and I think it comes with a single-player variant included.)