Bruce only thought he wanted Rachel because she was (admittedly) his oldest friend. But even she didn't understand the depth of his anger and his brokenness. She obviously knows about it, but she didn't understand it. In the end, she can't even believe that he'll ever move on. That's a pretty terrible conclusion to come to with a close friend.
I actually thought that in TDKR, he only got with Miranda because she lectured him like Rachel used to do.
He didn't even really notice that he was having fun banter with Selina!
Selina comes to understand where Bruce is coming from, that's the main difference between her and Rachel. And I think that understanding is why the deepest connection is between Selina and Bruce.