How would you hack Bitcoin?

There's been a lot of talk about bitcoin being a bubble. Putting that aside, what I'd like to know is how would you (or we) hack it?

From what I understand, the biggest flaw is it's peer to peer nature. If we created 100 million clients with the transaction chain we could become an authority in the peer to peer network. Therefore we could rewrite the transaction chain. It'd have to be executed suddenly to work and require a plenty of CPU/Storage but that's readily available.

What other methods exist for hacking bitcoin?