I'd say Physics, since I avoided that like the plague.
Chemistry and Maths are hard until they suddenly just click. I loved maths since once it clicks it becomes second nature, so you hardly need to revise at all.
Biology I found came naturally to me, so I'm not sure if that's because it plays on my strengths or because it's just easy. I love processes with one step leading to another in a logical fashion, so biology was perfect for that.
Physics... I found that cold and heartless. Electric currents, particles and waves hold no interest for me at all. Astro-physics always sounded cool though.