1453.
It's a little more challenging than 1399 because a lot of countries start in a stronger position:
-Burgundy has Holland and some other provinces they dont have in 1399 (if we pick a later start date, Burgundy is gone, and a game without Burgundy just isnt the same)
-Aragon has Sicily and leads a PU with Naples
-Portugal has Azores, Madeira and Tangiers
-Castille has the Canaries and Ifni
-Venice has some Northern Italian provinces
-Papal State has Avignon
-Austria leads a PU with Hungary
-Ottomans have conquered Constantinople
-Golden Horde has been replaced by Crimea and Kazan, so easier time for Poland, Lithuania, Muscowy and Novgorod
And it's still early enough to have fun. We can pick a couple of NI's early on, Reformation and Colonization is about to start, etc.