Founder is usually best. The other options available:
There are official titles that you have to chose when incorporating, typically you are a President because someone needs to be. But on business cards and email signatures Founder is best. I feel like you reserve a CEO title for when you have at least one other C-level person on a team (like a CTO) and an actual team, otherwise it's a bit misleading. When you have large co-founding teams, use of VP titles makes sense to differentiate roles and responsibilities of the co-founders.