According to doctors and other professionals. The best time is around 3 to 4 years. Because it allows your body to fully heal from having a baby and having all those nutrients taken from your body. It takes time to regain all of that.
I would say, personally, at least a year or two.
I also think people are saying around 3 or 4 years is so that you only have one in diapers at a time. I don't know. I do know that I wouldn't wait more than 4 years though (if you can help it) because the distance between them may feel a lot farther apart, with interests and things like that.
At the end of the day though it's your choice and your decision. Along with SO