I don't have a favourite. I love and hate different aspects of all my A Levels:
Maths - Love: When you finally understand a topic and it all 'clicks', you feel like a genius for a couple of minutes.
Hate: Trigonometry.Chemistry - Love: Really interesting knowing what happens at such a small level.
Hate: The tedious coursework and the 'wall of text' questions on the papers which have really specific corresponding markschemes.
Physics - Love: The look of admiration on other people's faces when they find out you study it - everyone who doesn't do it seems to think it's 'so hard'.
Also, understanding what happens around you is really cool, and so is seeing the real life applications of it.
Hate: Tedious coursework.
History - Love: I have a weird fascination with Nazi-era Germany and World War Two Britain which, luckily, are the two topics my school have chosen to teach us!
Hate: Having to memorise all the dates, names, etc.