Another vote for 2.5 years as the ideal gap - my toddler absolutely LOVES the baby and tries to help with everything

I also have a 7 and 9.5 year gap here. 7 is so-so, you get moderate amounts of help from the bigger one while the small one is a baby, then a year or so of the big one using the little one as a toy, then constant arguments when the little one suddenly sprouts a strong will and doesn't want to be a toy anymore but wants to play with the big one's stuff instead. With the 9.5 year age gap the two of them might as well not be related, the big one shows almost no interest at all, and probably will never change - they're opposite genders too. But its early days yet, we'll see.