High school: Physics or Calculus. Loved Newtonian physics, and I was always good at advanced math (sucked at basic arithmetic for some reason, always got bored and skipped steps)

College: Thermodynamics, Heat Transfer, or Fluid Dynamics. Combination of advanced math and physics, always really cool to me. Differential equations was a close fourth.

I liked my moral and ethical philosophy class, up until the professor started evangelizing to us about religion, which kinda ruined it for me