I have 3 years exactly between my two daughters. We would have liked a bit smaller of a gap but it took over 6 months to fall pregnant second time around.
They are now 4 and 1. Although my 4 year old is very quiet and gentle and my 1 year old is as rough and tough as they get they do get along really well.
My eldest dropped her last day nap when I was in the throes of morning sickness and that was really tough, but other than that the age gap has been great. My eldest was old enough to understand if I was feeling tired or sick, I could explain to her why I couldn't carry her or why she couldn't jump on my tummy. We could also help prepare her for the little ones arrival and talk to her about what to expect while I was in hospital so they are all the positives.
The negatives are that they will have very different interests due to their age gaps for the first few years. DD1 gets frustrated when DD2 comes along and interrupts her game of dolls or pinches her pencils. I don't like how they won't spend that much time going to school together. Especially as DD1 will be starting prep as a 4 turning 5 year old and DD2 could possibly start as a 5 turning 6 year old!
They are the only negatives though. I don't really know if there is a perfect age gap, everything has its positives and negatives