I can't speak from experience, but I've been trying to figure this out myself by looking at other people's children...how easy/hard it was for them to take care of them and how the children behaved as they grew up...how they got along and how they socialized with others.
A lot depends on your preferences and parenting style.
Some parents choose to "get all the diapers over with" and have their babies 2 years or less apart to get all the baby stuff over with. It seems that these babies, in general, are great playmates when they're young, then fight constantly when they're school age. They usually wind up being close again when they're adults, though. I personally do not want to go this route as I don't think I could deal with 3 kids 3 and under, as some of my friends and family have.
Other parents want to "enjoy" each child indiviually and wait until after the first child is in school to decide if they want another. in theory this gives each child time to be the center of attention...the eldest is the star when he is young, and the youngest is the star once the other has gone to college. Also the eldest child can help care for the youngest, and no one is in diapers or still at home at the same time. I don't want to go this route either...my brother and I (as are my best friend and her brother) are 7 years apart and though we never fought as kids, we were also never very close. He spoiled me and I adored him...and then he moved away when I was 11 and we drifted apart. Same thing with my friend.
I'm going to aim for 3-4 years apart with my kids. That way I won't have any in diapers at the same time, but they'll be close enough in age that they can share some activities...just not all of them to the point where they feel like they're not individuals anymore. It seems like kids this distance apart fight when they're young, then become friend in high school and adulthood. I have a friend whose 3-years-younger sister is honestly her best friend, and my partner is very close with his 4-years-younger brother.
So to me this is the ideal age difference, because of what I've seen in other people's lives and what I think would work best for me, and it seems to be the best age difference for the kids to not be so close in age that they drive each other nuts but close enough that they can relate to each other, but other people may have different opinions and different parenting styles that may make another age gap better for them. but I plan on starting to try for another a few months before my son turns four, if all goes according to plan.
sorry such a long response...i've been thinking about this same thing lately