I had 2yrs 6 months between my first two and it was really hard. The two yr old was not old enough to understand having to share me and was not as independent as I thought they'd be by then.
I think a very small gap or a gap over 3.5yrs is best. If the gap is very small, (like under 18m) the baby/toddler knows no different and just gets used to the younger sibling being there by the time they are at the age of jealousy. Plus by that time the younger baby is not a newborn anymore so is less demanding (that's my theory anyway).
If the gap is bigger like over 3yrs the older child seems more likely to dote on the baby and is that bit more independent and can handle the change better.
This time my youngest is 9 and by the time baby number 3 is here I'll have a 9yr old (nearly 10) and a 12yr old!
Slightly bigger than I'd have planned (LOL!) but I do think it has its advantages. They are able to occupy themselves and can make themselves snacks, breakfast & sandwiches etc when I've been busy or not felt up to it (with nausea or tiredness lol), and my 11yr old will go to the shop for me if we need something, they are definitely a big help at this age.