I lean toward "The Hidden Fortress" because the rebel base seems to be the central element of EP IV. The Imperials seek out the rebels Hidden Fortress so obsessively that they even allow Luke Han and Leia to safely deliver the Death Star plans to the rebellion, just so that they may find the base.
They could have stopped them from ever escaping the DS and it all would have ended right there, but as Tarkin said, they took "an awful risk" all to find the Hidden Fortress and their consequences were disastrous.
The only one that may not get retitled is ESB and that is because of A) my love of the title, and infinitely moreso B) I haven't heard a single great alternate title for ESB, nor have I thought of one myself.