Assuming that “best” in the OP means “most realistic”, I’m going to vote, among other things, June 17th, Petain’s request for an armistice. Britain now realizes that her ally is basically defeated, but France has not capitulated yet, so the German government isn’t negotiating from a position of power. If Petain and the British can contact each other than there’s a fair chance that peace talks might begin. Petain’s anti-English stance might derail this but his major interest is to preserve France and right now London can help him do that.
I also voted:
- Capitulation of Belgium (because this kind of set in motion the whole surrendering train that would continue until May 1941 in progress, so it shows that the Allies cannot win this war on their own).
- Italy joins (Mussolini had an interest in hosting a peace conference so he could decide to make a serious proposal to the Allies instead of entering the war)
- Battle of Britain begins (This is the practical beginning of the stalemate period outlined in more detail by DaveBC).