None of them is a must anymore when you meet Her Majesty the Queen. In the photo above, former British PM Theresa May curtsies at the beginning of an audience. It is the traditional way: men can greet the Queen with a neck-bow while women do a small curtsy.
Nowadays a usual handshake is formal enough, however, etiquette says you must wait for the Qeen to offer her hand and than shake it politely.

GR8 to Know
To address the Queen in a formal way is that you call her “Your Majesty” and then “Ma’am”.

Photo credit: WPA Pool / Getty