I can't think of a game that is CPU heavy on SP mode. CPU comes in play mostly when the game requires some sort of logic calculation such as determining complex AI behavior. So the most demanding SP games would be ones that feature a lot of AI in them where the AIs have a lot of different behaviors. I'm not talking about 5-10 AI, a lot more than that.
Whoever said that GTA 5 is CPU heavy : While GTA 5 has a lot of NPCs, they do not require complex calculation since there is only so much interactions they have.
If I were to think of a game, probably Civilization series is going to need a lot of CPU work and this goes up the more ennemies you have in a world.