My favourite single player games include the following in no particular order:
Super Robot Wars (many platforms, Turn Based Tactics)
A fun series of games, unfortunately most of the them are Japanese only aside from a few fan translations and OG1 and 2 for the GBA. I prefer SRWOG series to the others since I don’t watch anime and thus have no idea who the whole cast is in non OG games.
Caves of Qud (PC, Rougelike)
A nice game where you can play as a Mutant in a bizzare post-apocalypse world. The amount of
crazy fun stuff you can do here is outstanding!
Cataclysm: Dark Days Ahead (PC, Rougelike)
Another great game set after the Apocalypse, although unlike Qud this one takes place in our familiar (mostly) modern world. There are heaps of things to do and find, the only downside is that recently some of the crazier fun stuff got removed ( like the ability to make Mini-Nukes for your personal use)
ADOM (PC, Rougelike)
The last Rougelike on my list, I liked it less then the other two. It is still very nice when you get into it but It is almost impossible to do so without spoiling yourself silly
Age of Wonders I and III (PC, Turn Based Strategy)
For some reason the Age of Wonders II didn’t stick with me, but I really liked the other two even if they did have some issues in the lategame (constantly spamming the best units gets annoying).
Avadon Trilogy (PC, RPG)
Tomb of Tyrants (PC, Three in a row? the genre is mixed and mashed here)
A fun game where you play as a mighty Tyrant who hides at the bottom of his dungeon and commands his minions to stop adventurers from getting to him.
Special notice to Wonderland Adventures series, cute puzzle games for PC that give me nostalgia