There are two years (and one day) between my oldest two, nineteen months between the middle two, and just over two years between the youngest two. Non of the gaps are particularly big, or that different, or at least that's how it seems at first. In reality, there's a huge difference between the two year age gap and the nineteen month gap.
Two years is good, they're really close. Perhaps not as close as the middle two though, with their nineteen months. The things about close age gaps though, and I suspect this is even more apparent if you have an even closer gap, in the early days it's hard. Any thing less than two years, and to begin with at least, you have two babies. And that, is draining.
It gets easier though, children who are close in age go from being very very hard work, to playmates who will entertain each other quite well. Somewhere in the year or so after the second one arrives, things become easier, and although you don't notice the change, at some point you suddenly realise that it's not as hard as it was.
In my own family, there's quite a big age gap between me and my brothers. Nearly five years between me and younger brother one, and six between me and youngest brother. When we were younger this age gap was enormous. I was off to school by the time they came along, and on the few occasions where we were at the same school it was only brief before I moved on. These days though, it doesn't really feel like such a gap, it's only when children are little that you notice these things. So a perfect age gap? I'm not sure that there is one, it doesn't really matter.