Well I've tried a lot of sweeteners. Sugar, Aspartame, Acesfulmine K (or however you spell it), Sucralose, and Saccharine all do not work for me. I usually feel cruddy in one way or another after using any of these.
The only sweetener I've found that I have no reaction to is Stevia. Here's a quick description of the herb.
What is stevia? A shrub native to Paraguay, used for over 1500 years by the native Guarani Indians. A herb with an abundance of positive effects. The whole leaf contains numerous phytonutrients and trace minerals and is much sweeter than sugar with out negative effects. It can sweeten any drink with no calories, no carbohydrates, no tooth decay and is diabetic safe. Stevia nourishes the pancreas and does not raise blood glucose levels, making it not only safe for diabetics but also beneficial. Since there are no calories or carbohydrates, stevia is an excellent weight loss aid. It does not contains the negative side effects reported with the use of artificial sweeteners including aspartame. Adding stevia to your diet on a regular basis, it has also been reported that it can help minimize hunger sensations, cravings for sweets or fatty foods, aide in digestion, decrease hypertension without effecting normal blood pressure, stabilize blood glucose levels, shorten recovery time from cold and flu and aide in addictions to tobacco and alcohol. When used in toothpaste or mouthwash, cavities and gum disease is reduced due to its antibacterial properties.
This is just a description I took from Stevia Canada's website. I don't have studies to back up any of that nor am I associated with Stevia Canada. I also don't want you to buy Stevia from me or any such silliness.

I just thought I'd share my experiences using this herb. It's very sweet. A heckofalot sweeter than an equal volume of sugar. I mostly use it in liquid form. I'll often make Kool-aid with it. It tastes great and I don't feel like crap after drinking it. Life is good.
It's also good for sweetening tea and coffee. I'm not much of a coffee drinker but my girlfriend is and that's what she uses.
I also posted a no-bake chocolate cheesecake on the recipe forum. Just had it this weekend and it was sublime.

It has a lot of uses and I'm not going to go into them too much. There are a lot of stevia recipes on the web that can be tapped and tried if you're interested.
I just thought I'd put up a little note about my positive experience with this sweetener. I'd also be interested to hear anyone else's experiences with sweeteners and hear other opinions on all things sweet. =)
I'm a low-starch dieter, but I've found that it's very important to control sugar intake as well. I hope this can be useful for some of you out there.
Take care.
Cody.