Definitely gym classes - Body Attack, Body Combat, RPM, Body Pump, Kickboxercise
I'm not a runner, in fact I'll avoid it if I can!! I was asked today what the best exercise to lose weight was, and I replied "Cardio to burn the fat, weights to tone the muscles underneath, but whatever it is you do, make sure it's something you enjoy. If you enjoy classes, go for it, if you love running, do that. You've got to like it to keep doing it!!"