Any kind of pad/bag work will help out your cardio but you ahould also run and/or swim to keep that at a high level. Pad work is great for working on combinations, speed, and accuracy. A heavy bag will help with your power. My favorite bag is definitely the double end bag. It is the best for working on your accuracy in my opinion. It helps your speed, as well as your footwork if used properly. A speed bag is good for timing but I am not a huge fan of them.