Is there any form of cardio that is better than another? Where does using an excercise bike rank? Any exercise that is designed to increase or maintain your aerobic capacity is a good cardio exercise. This can be anything from jogging and cycling to jump rope or walking. The key is to keep above your threshold of training (within your fitness target zone) for an extended period of time (20-60 mins depending on the activity). Activities of a lower intensity would need a longer duration of time.
An example, on average running is of a higher intensity than cycling, so you need to cycle longer to match its effectiveness to running. Whatever activity that you enjoy the most that is of 20-60 mins in duration and of an intensity higher than your threshold of training would be a good cardio/aerobic exercise. What I might find as the best activity for me, may not be the best for you. Keep in mind any injuries or ailments that you may have. Someone with a bad knee may not be able to run pain free, so cycling would be a great alternative. With all this in mind, my advice would be to alternate various aerobic activity.
Try running one time, and cycling another. Walking, aerobic exercise machines, aerobic interval training, dance, hiking, backpacking, cross-country skiing, and circuit resistance training are all great alternatives to try. This will prevent you from getting stale/bored with your activity and ensure all around fitness. One last note that may be just be a given, but it is always wise to check with your Doctor before starting any exercise, especially a cardio program. If you haven’t been doing any exercise for a long time, and jump into an intense cardio workout, you run the risk of damaging your heart/lungs. Remember, your heart is a muscle too, and it can sustain damage if it is highly overworked.