It really depends on what kind of game you're looking for. There are a lot of very good single player games out there, and what's right for one person might not be right for another.

Here are some questions you should keep in mind for selecting a game (besides budget): Are you looking for something lightweight that doesn't take up a ton of room and can be played quickly? (Friday, Onirim)

Are you looking for something heavy where you have to spend time thinking about each action? (Mage Knight, Spirit Island)

Is this a game that will be sitting out on your table for weeks? (any campaign game)

Are you looking for something with a story/adventure? (Tainted Grail, 7th Continent, a lot of other Kickstarter games)

Do you want something that has a ton of expansions coming out all the time? (Arkham Horror LCG)

What genres are you into (sci-fi, grimdark fantasy, historical, etc...)?

As mentioned in another comment, the People's Choice top 200 by the BGG 1 Player Guild is a very good resource, but I wouldn't necessarily focus on just the top 10 (for example, I'm not a fan of Spirit Island or Marvel Champions, but I know other people are).