My parents, for sure. They're pretty much all we've got. DH's parents are gone and our brothers (mine and DH's) are far too young and stupid to be caring for children - I won't even let my brother babysit either of my DDs and DH's brother would never ask (he's just 18 and can think of far better things to do with his time than be around toddlers - riding his motorbike and drinking with his mates over a few video games are just a couple of suggestions). In the future, I probably would still prefer my parents over our siblings as I just don't ever see my brother getting his act together (nor would I want my girls around his partner for any length of time...), and if BIL keeps up the way he's going with avoiding contact with us if at all possible, he will be a stranger to his nieces
Luckily my parents are still relatively young and have a great relationship with the girls - they see nanna and poppy at least 3 times a week, usually more, and go for sleepovers once every couple of months (it's usually pretty frequent on school hols when Mum's off work for a few weeks). They wear my parents out and while I may not agree 100% with their parenting philosophies, I know my olds would do a much better job raising my babies than anybody else I can think of. And really, when your choices are so slim, parenting methods/philosophies are certainly not at the top of my priority list in choosing a suitable guardian!