Swimming, because I'm not young...it's something just about anyone can do! and you have so much fun.
Plus, it matters not if the weather is hot. You get the best of both worlds....so swimming has to be for me and I know many many people, the ultimate in exercise.
My favourite form of exercise is when it is something enjoyable that doesn't essentially feel like exercise. So for me, that could be swimming, kayaking, cycling, zumba, dancing, roller skating or climbing. For ease, I run regularly as I can just leave the house with no gym pass required, but for enjoyment factor, it would be something a bit more fun.
I like feeling my muscles work, and boxing (not boxercise) does just that for me. It's anaerobic, meaning it burns much more calories and energy for the same amount of time as other workouts. It's intense and efficient; it tones, strengthens and conditions. That said, I also enjoying swimming and circuit weights at the gym.