I think all of my kids are close, but if I had to say - my two oldest, who are 21 months apart, then the next two, who are almost 4 years apart, and then "the babies," who are 9 months apart. They all get along well, though, and love it when they ALL get to hang out together. My dils are wonderful about wanting the kids around, too.
If there is conflict, it usually involves my 15yo dd and my 13yo ds. They are 19 months apart.