nobody mentioning yoga?? Definitely the fastest way to lose weight ;) visible changes after every session! I have to feed my yoga sims cake to stop them from being skinny lol
I find pushing the limits either jogging or running on the treadmill to be pretty effective too. Swimming is the slowest way for sure, I started a max overweight sim off in Sulani, and after maxing fitness skill through swimming everywhere, I could barely notice a difference in his weight..
The food and weight system is surprisingly deep considering the rest of the games mechanics. Each meal has different caloric content too, so your sim won’t get fat eating salad, but will eating cake. I used to get my sims making grand meals all the time as it took them longer to cook, so they’d build cooking skill faster, but they’d also end up fat from eating grand meals all the time lol so watch out what you feed them ;)