In curiosity, I know all of these can hack the price of Bitcoin, but the question is, which in your opinion is most likely to happen first?
I honestly can't for the life of me imagine how the blockchain would be hacked, other than a 51% attack or whatever it is called; I suppose double-spending can be seen as a flaw, but that was more on the implementation into a market/commerce settings being uninformed and naive. I think there are some people worried about the SegWit implementation because I think that introduces some new security flaws to the blockchain, we shall see.
Government bans would just increase the price IMO, when your government tells you that you can't have bitcoin... That's when it's really time to get some bitcoin.
Exchange hacks have happened in the past and destroyed the price of bitcoin few a little while if I remember right. MtGox had a really bad attack that chunked the price of bitcoin at the time because most transactions on the bitcoin network were associated with MtGox at the time. Er go, my vote is on Exchange attacks, because they've happened before and I wouldn't put it past an exchange to try to make a quicker buck at the cost of their security, being ignorant to what bitcoins capabilities and limitations actually are and only being concerned with how much fiat they can make day-trading.