Well, if you want to get in shape the quickest and have no problem with your back or knees then put on your running shoes and hit the street! Running on the street is the fastest and most effective way to get in shape, coupled with resistance training of course.
If you have back or knee problems then you should either do a bike or the elliptical. The standing pieces of cardio will shape you quicker like the elliptical, arc trainer or Precor AMT. These are what we call non-impact trainers because you do not lift your foot to do the movement. But because you are standing they have a tendency to shape you quicker than the non-weight bearing cardio pieces like the bike or the UBE.
The other piece of cardio equipment that is very effective in improving your cardiovascular system and having you lose weight is the moving stairs. Try this for a short time and then work your way up to 30 minutes because it is a hard workout that produces great results!
I recommend moving from one piece to another to keep your cardio workouts interesting and keep your muscles guessing. Do 15 minute on one and then move to another and then to another for a combined time of 45 minutes. That is your goal for cardio: 45 minutes!
Good Luck and Keep following The FIRST Rule of Fitness – SHOWING UP!!!!