It should be the 2013 borders. Before the invasion of Crimea. Russia agreed not to invade Ukraine, in exchange for Ukraine giving Russia it’s nuclear weapons. If Russia doesn’t give everything back, then it can’t be trusted to make any deal with any nation. Including peace deals.
Any agreement needs Ukraine to get all it’s territory back, and allow them to join NATO. Russia can’t be trusted. But they won’t challenge NATO directly. Europe also needs to take defence more seriously. Only eastern Europe is really taking the Russian threat seriously. Western Europe was fooled into thinking they won the cold war. Russia and China never saw it as over.