It's really cool that you're so into math; it is important to have a grasp on math if you want to make games (Not level design or game design, per say, but actually code a game).

I would say algebra or calculus would be the way to go, but more specifically

Linear Algebra. Linear algebra is really useful for 2d graphics and absolutely essential for modern 3d graphics. Vectors are often the best way to represent position, speed, and rotation, so understanding how they work and what you can do with them is really helpful.

Linear Algebra is usually taught at the college level, though, so it might be a bit difficult to find easy to learn from material. I think Khan Academy is a great resource for linear algebra, but was a bit tedious (IMHO). However, if you haven't learned this kind of stuff before, it's really great.

EDIT:

Trigonometry might be a bit of prerequisite for linear algebra, so definitely learn some of that. Also, a full understanding of linear algebra is not needed to help you with game programming, just learning more about vectors will be really helpful.