I think that what is "best" differs for everyone.
I have a friend who planned her babies very close together - about 15 months apart. I thought she was crazy, but didn't say that, of course! For awhile, she told me it was crazy. She seemed exhausted, distracted, always off-balance. Then, when her boys were about 3 and 4 years old, I saw the beauty of the small age gap. She was done with nursing, diapers, strollers - all the baby stuff. The family seemed really cohesive. The boys could attend the same extra-curricular groups. Generally, there seemed to be less running around, easier organization etc. with 2 kids close in age.
We have a 3 year age gap and I like it. The older one was entering a period of independence when his baby sister was born. Not only did he not need me as much, he didn't want me to be with him always. He was developing his own friendships and social network. He wanted to dress himself, feed himself, etc. It made the transition from caring for 1 child to 2 much easier, I think. They "play" together quite well, although now that they are 17 and 14 y.o., that means that they share music and books (and occasionally clothes!) and he takes her with him sometimes to listen to bands he likes. They have a good relationship.
I had my first after the first year of law school and took the next year off to be with him. (Law school wouldn't agree to part-time schedule, but that's another story!). I returned to a combined Law/MBA program and had my second just after finishing exams in my final year and graduating. I've always said it's easier to combine motherhood and university than motherhood and a job. Most profs are fairly flexible about assignments or substituting a paper for an exam when family demands interfere. It's much harder to explain to a boss or a client why deliverables that they have been counting on aren't on-schedule. Perhaps you are continuing in the ivory tower of academics after you finish your PhD, but if you intend on moving into industry, you may want to think about it a little. Particularly if you are somewhere with lousy maternity/parental leave benefits.