I can't pick a cl: Kickboxing, Krav-Maga, or MMA?

I guess mma would give you the best of all worlds. Being more of a sport they should include a lot of conditioning work like cardio and resistance exercises. Some mma techniques were originally also used in female self defense cles. Remember MMA uses a lot of kick boxing as well as wrestling and ground fighting all of which require a lot of energy and fitness.