What he feels for Shanks is "admiration", completely different from what he feels about his nakamas and brothers.
Shanks would be the last. The others 2 are equal. Luffy wouldn't be able to decide such a thing.
he likes brothers more...he literally collapsed when Ace dead and didn't recover until Jinbei reminded him about his nakama....but ofcourse if both his nakama and his brothers are in the same predicament, he choses strawhats
I think he loved Ace more than anybody. He grew up with him, trained with him, was looking up to him and spent more time with him than with Sabo for example.
Next comes some of the straw hats (the first members like Zoro, Nami, Usopp and Sanji then the ones who joined afterwards, Chopper and Robin, then Franky, and finally Brook).
I don't where I would put Jimbei, he seems to like him a lot.
Sabo comes after East Blue members but before the others.
SHanks is last.
But he loves them all very much. So in the end it doesn't really matter.