Which is the best exercise to have a toned tummy?

I want to maintain a healthy weight, because I am 12 kgs underweight(my weight is 46 kgs, but my doctor advised me to gain some weight because I am actually supposed to weigh at least 58 kgs - considering the fact that my height is 160 cms). I am thin, but I have a really shapeless belly. I am eating healthy now and have started going to the gym, but I wish to know which exercise machine is the best for losing the out-of-shape abs and love handles, and which exercise(without machine) is the best and fastest means to get into shape. I want to tone up my tummy, I don't want my diet to add even more fat over there. I hope this is not a weird question...