DS1 (just turned 5) has a 'best friend' He is 6 days younger, lives 200 metres away, and both he & DS1 have been together at nursery since they were 6 months old. They just started school together and sit on the same table.
However I can't help feeling this is part due to circumstances rather than choice (although they DO get on well). We see the parents a lot too, and our DS2's ( aged 2) also play well together.
Since starting school he has also developed a much stronger friendship with another little boy we knew previously, but didn't see that often.
My observation is that children do seem to do a bit of intuitive 'self - sorting' - getting together with other kids who seem most like them in terms of background & abilities? DS1 has lots of other 'friends' he talks about though, and they all seem to fall in & out of favour.