1 Part Water
1 Part Sugar
(I personally prefer 1.5 parts sugar)
The higer ratio of sugar versus water will create a colloidal mixture making it a thicker yet almost tranlucid smooth liquid, softening the flavors and facilitating the mixing process when shaking/stirring your cocktails.
Not so simple syrup
Ascorbic acid or citric acid will reduce the presence of any possible bacteria, not onlt that , but it will give it a refreshing taste. Squeeze half a lime or half a lemon per every liter of simple syrup.
Confectionery sugar versus refined sugar
Confectionery sugar allows for a higher ratio or sugar to water (3:1) when you are diluting the components.
Distilled water versus tap
Tap water contains some purifying agents and additives that might be noticeable in the final taste. It is worth to use distilled water instead (filtered water might work too) when possible.
Less than a drop of Vanilla
Add one drop of vanilla per every 4 liters of simple syrup. if you are just doing small batches of simple syrup, you can use a simple smear of it. More a drop every 4 liters will be very noticeable since we are talking of liquid than mixes two very basic flavors.
Strain and Bottle
Cooling your mix right is very important. Set it aside until room temperature. If you need to speed up the process, then put some ice water in your sink and place the pan on it, to speed up the cooling process.