People get very attached to particular cardio machines, I've noticed. One of my pet peeves is seeing people come to the gym and get on the same machines and do the same workout every time, especially, when they are using the least efficient machines there. It seems like such a waste of time because they are unlikely to be getting much out of their workouts after a very short period of time.
First off, you should be changing up your workout as often as possible. Your body adapts to the demands placed on it very quickly. So if you aren't able to increase the time or intensity of your workout, at least change another variable. Alter the incline on the treadmill, do intervals instead of steady state training on the stairmaster, etc.
Second of all...and I know I'll piss people off with this one, unless you are injured, pregnant or over age 60, get off the elliptical!!!! They are the least efficient machines in the gym and they do not mimic any real life activity. Most people (there are some very self-motivated people for whom this may not stand true) do not push very hard on these machines and they are inherently less intense than some of the other machines because they are not weight-bearing. If you can do weight-bearing activities, you should, because they are generally more efficient calorie burners and they will help to strengthen your bones. I feel similarly about stationary bikes. If you are able to do something else, do something else.
My recommendations are:
#1 - The treadmill. Running, of course, is one of the most efficient activities out there. If you can't run, however, walking is equally as good. Just make sure you walk as fast and on as steep an incline for as long as you can.
#2 - The stepmill. This is like a treadmill with rotating stairs. They are massive, tall machines, made by the Stairmaster company, which can be found in an increasing number of gyms. These machines kick butt! They are also low-impact, however, some people with certain types of knee problems, find stair climbing painful. If I could afford one and had the space in my house, I would totally buy one of these, they are fantabulous. I think the reason they are not more popular, is people find them intimidating. Don't! If your gym has one, USE IT!
#3 - The Stairmaster. Now I am talking about the thin, upright machines with two pedals. Personally, I only like the ones made by the Stairmaster brand, but there are probably some other decent ones out there. Just don't lean your weight on the machine!!! Same goes for the stepmill, by the way.
So if you are a gym goer and you are trying to make the most of your time at the gym, these are my recommendations to you. And, of course, don't forget strength training and flexibility training as part of your fitness regimen.