It depends on how much you need - which is decided primarily by your tank size. Other lesser factors include how efficient your dissolver is and how much surface tension you have.
Before I recommend you a mixture, please tell us your tank size, the method to dissolve, how much surface tension there is (spray bars, hob filter, airstones...that kind of stuff), and your watts of lighting. I've had a great experience using my aquaclear 150 as a dissolver, just to give you an idea in case you are unsure on the dissolving method.
I've been experimenting alot on this mixture dilemma, and here are my observations that will benefit any mixture:
1) Make the solution as dilute in sugar as possible. This means cutting down on sugar and/or use more water. There is of course too little sugar that will not provided sufficient fuel for the yeast. I like to add 1/2 - 1 cup of sugar to a 2-liter bottle that is filled with water 2 inches from the top. Most mixtures recommend 2 cups. This is too much because alcohol builds to fast, kills the yeast, and leaves lots of unused sugar.
By doing this, we have a longer lasting mixture and even CO2 rate; not a quick burst of CO2 that lasts less than a week.
2) Of course, by having a lower concentration of sugar, the rate of CO2 production decreases as well. To amend this problem the best solution (pun not intended) is to A) add more yeast or B) use more bottles.
3) No baking soda. I have read in a few places that sodium from baking soda kills yeast, and at the same time yeast do not appreciate extra buffering - they like acidic conditions. While this isn't a widespread fact, I have noticed that baking soda has decreased the life and efficiencies of my mixtures, so I don't add it. In case you have very poorly buffered water, maybe baking soda can do more good than bad since having water become too acidic is bad. But if you decided to add some - NO more than a pinch. Adding the common 1/4-1/2 teaspoons sends KH skyrocketing into the 20 degree range, which also is ridiculously high for Na.
4) Temperature has an effect on production rate. Higher temperature = equal faster rate and vice versa. So for more extreme room temperature like 78F and higher or 66F and lower, the mixture would need to be slightly adjusted too correct the rate.
Here are guidelines that can get you started on your quest for the "best mixture." There is no best mixture for everyone, but it largely depends on your own situation. After providing the info I asked for, I could give you a ballpark mixture to start. After that it is just some adjustments that you can do which will make it the best mixture for you.