my sister and BIL are ours. they are ds' god paretns as well. but the reasons: my brother is single, young and working in the mines and would be unsuitable (although i know he would provide financial support if anything happen ); my SIL is young, single, living hand-to-mouth and has childhood diabetes; our parents are getting on and have raised their kids.

So that leaves my sis and BIL. But they are just teh last ones standing:
they are loving with my children;
their little boy has just arrived after 9 yrs of trying so the kids will be close in age;
my sister 'does things' like me;
they are financially secure and both have careers taht will see them advance further;
they live near my parents and family;
they get along wtih my ILs so i know there would be no issue with my kids staying in touch with the other side of the family;
and i just know how much love and support they would give my kids. and I know they'd be brought up to 'know' dh and i as well.
they would give them all they need emotionally and financially and would never make them feel like they weren't "theirs'' iykwim.
My sister is afirst time mum so is finding her feet and will do things parenting-wise that will be different to me, but i know her heart and that all that matters to me

good luck, it is a big decision that hopefully none of us will ever need to actually have used!