It's changed a lot over time. (Mostly because I've shifted between martial arts quite a bit.)
Currently, I've found the best stance, is the one I learned from Bas Rutten's Big Book of Combat Volume 1. It somewhat resembles the forward stance that you take in karate when throwing a punch (but a little less exaggerated). Both your toes are pointing forward, your back heel is off the floor, and your front leg is bent somewhat more than your back leg. You're completely squared off to your opponent.
The small change I've made to it is that I keep my elbows a little closer to my body than Bas does. But either way, this stance looks a little awkward at first, but actually it's worked better for me in MMA and Stand Up sparring than any other stance. It gives you way more power in your lead hand, and it helps a lot with kick defense.
The only time my stance changes is when I'm in boxing range. Sometimes to throw my opponent off, I move in to the boxing-only range, and I use the hunchback stance as taught by Rodney King from the Straight Blast Gym. You roll your shoulders forward, hunch your back, and keep your gloves on your head. It makes you really difficult to hit, and it really upsets your opponent when you keep throwing hooks and uppercuts at him while he can barely get a good shot in.