My first two are 17 months apart, my favorite gap then 3yrs 10 mths between 2nd and 3rd (they do not get on). 22 mths between 3rd and 4th, also a great age gap, they get on well.
Then I remarried so after 8 mths I had my last three, who are 24 mths apart...DD2 had just turned 2 when DS4 was born.
11 mths between 5th and 6th (born in the same year) and 13 mths between 6th and 7th.
I have found it a little hard at time with the last three so close, it can be very full on but they are close and I think particularly the girls will be best friends.
One good thing is that they go to school one year after the other, so they will be in year 1,2 and 3...so they should be close at school too.