Obviously tennis will be the first choice of most, but why do you love it? Personally, my favourite sport of all time is test cricket, just because of the intricacies - the conditions play such a major factor like pitch condition, overhead conditions, size of the ground, and they vary from ground to ground. A test match is a story full of twists. And it goes for 5 days. I love the history of the game, the rivalries and the mental strength and expert skill required to play. Tennis is a fantastic game because its the best individual sport out there. Its a combination of mental strength and individual skill. Again conditions play a big factor (eg: court surface), and to be the best isn't just about how athletic you are, its mainly in the head. I also enjoy the different strategies and tactics you come across. I wish serve/volley was still around.