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Well, according to some Lifetime thing I read last year the treadmill will give you the most bang for your buck. I always feel like I'm not actually doing anything on the exercise bike unless I have the resistance turned up ungodly high. If I were going to rank them:

1. Treadmill
2. Stairmaster
3. Exercise Bike

The treadmill can be hard though if you have bad knees or ankles, but the impact does strengthen your bones as well. I think the biggest con to this piece of equipment isn't in the machinery itself but that people feel a need to keep up with others on the treadmill around them. Or the embarrassment of trying to figure out the controls a bit but if you can get over that and go at your own pace this is a nice option.

Stairmaster will probably give you a nice butt and strengthen your quads and hamstrings more while getting your heart rate up. I personally don't care for them, maybe because I travel up and down five flights of stairs several times a day anyway just from being at college.

Exercise bike... if you can get your heart rate up on it good for you, but I have a hard time getting it there and maintaining it without putting an unenjoyable amount of resistance on it. I find it being my "lazy day" equipment, or when I really need to study but want to make sure I go to the gym too. The seats can be uncomfortable. I just get so bored on them. Maybe if there was a spinning class I would enjoy it but on my own it's my least favorite.

Any of them will be fine for weight loss if you get your heart rate up while using them. They'll tone different bits of you but overall you should lose weight on any of them. The exercise bike is good for strengthening your lower back, so if you have back problems or hip problems then that could be a good idea, since you can lose weight and help with that at the same time. I usually use the treadmill, just because I like running, but on days when I get bored i'll do 15mins running, 15 mins cycle, 15 mins elliptical

A couple years ago I lost about 30 lbs (from ~190 to ~160) by playing 30-45 minutes of heavy mode every single day (and improving my eating habits). 3nodding I doubt it helps much with muscle tone, though.

Does the gym have a cross trainer? Its good to do alternations of different speeds and resistance on those, but then go back to an easy setting as a rest then work up again.

Example:

1 min on lowest resistance, keeping a medium speed, min 2 next resistance setting and increasing the speed by .5

Then continuing to work your way up to a hard resistance and speed then starting over from the beginning, Repeat the process about 3 times.