Inconsequenti-AL
Breaks Games
I would think that World of Darkness could be used to do some really good solo work.
Superhero games, in general, often work well for solos.
I would second both those things...
Also have a pet theory that you could run an awesome cyberpunk setting game with 1-3 players - get some really interesting plotting going. Lots of good systems for that sort of thing out there. With just 1, figure recreating robocop could be fun.