Every muscle is important, kinetically speaking. The muscles that will obviously require the most O2 will be the thigh/glute regions. Everything has a pro and con to it, but the most important thing is flexability because of the stretch reflex. The longer you can stretch the muscle, the stronger the reflex will be, thus more power generated. Cardio is a hard thing to say because some people, like myself, are built for cardio (e.g. higher Hb, more dense capillaries, etc) and easily adapt to higher O2 demands. The main thing to focus on is to find a weight that is comfortable and easily maintainable. I float within 3-4 lbs of 185 year-round because I have reached a comfortable amount of mass on my frame. I focus on strength now and have gotten much stronger over the past 3 years without putting on significant LBM (maybe 3-4 lbs with some recomp added).