2y5m between #1 and #2. In this case though I think the fact that ds' birthday is the last day of the school year so he is 3 school yrs ahead of dd1 has made the biggest differenc to their relationship. Ds went to school when she was 18m old, if he had been born a day later she would have been 2.5 when he went. They don't have a very close bond, I think this at least contributes to why. They are growing somewhat closer now dd has started school, before the age gap has always seemed bigger than it actually is.
Dd1 and dd2 are 2yrs and 1 wk apart and much closer. Personality plays a big part too imo, dd2 and ds who are 4y5m apart have an easier relationship than ds and dd1.
Pros and cons in terms of parenting are too difficult to pick apart from dcs personalities, the slightly bigger gap was easier practically speaking but in hindsight that was due to ds being a physically predictable child. Dds are much more physically difficult but ds is the hardest child to parent by a big margin and it has always been so. If I was making these decisions again I might have considered leaving a little longer between all 3 because I have found it hard and restrictive. The other side of that is the baby part is all over and done.
Another thing I probably would say is that sibling relationships aren't set in stone, they're always evolving. None of my dc showed signs of jealousy or competing for my attention in the early years, that turned up later on and different things bring them closer as time goes on and each child experiences different things. It's great to see and makes me less bothered about the specifics like age gaps.