Hard to pick one. Kinda like asking which is the best tool - the screw driver, the drill or the hammer. Each one has its specific use. But if you want to pick a couple, I would pick the pushup and the pullup. The pushup has endless variations - wide, diamond, lalanne, feet up, pike, tiger, handstand, etc...to hit every muscle in the chest and shoulders. I can't think of a better lat exercise than the pullup. Couple these with crunches and you have the entire upper body, without any gym equipment to boot.