Entirely depends on the person. No A-level is easy, but my easiest would be either French or sociology. I did 10 UMS better in sociology at AS, but then I also did more work for it.
Seriously, remember 20 formulas and a dozen definitions. You have an A.
Ha! Ya see, I could do that... but physics is still beyond me!
My problem with physics is that my answer to a question varies according to time of day, number of fingers required for calculations, and the intelligence of the people whose test paper is within my line of sight...
Obviously, it is all subjective and dependant on individual strengths/weaknesses, as previosuly mentioned.
I personally find Geography to be the "easiest" subject that I have taken (excluding general studies) but that it is probably because I enjoy it the most. That said, I performed equally well in all of my subjects, at least at AS level, so I wouldn't say any were especially hard. It's just a case of how much you can remember.
For Physics, it really depends on which board you are doing. I'm doing Advancing Physics, where the content is relatively easy, but the question style is absolutely awful (Plus dreaded Section C questions, which are just plain stupid).