Tip the first central bank to announce stimulus or other market support post Brexit? Note - Plunge Protection Team does NOT count.
Could be a combination of BoE and ECB. Would only be guessing. For all I know BoJ has already intervened in the fx market to push down the Yen after it broke down below 100 against the USD.
I'm going with ECB to be the first to officially intervene to prop the Euro. Mario is going to want to get on the front foot and calm this storm down as soon as possible lest a panic spread throughout the eurozone and cause (further) loss of confidence in the EU.
...and the winner is....the Swiss National Bank! An unlikely winner, but technically an intervention in support of the Euro. Those central bankers are tricky mofos. Always hit you from left field....