St Peter's Basilica is probably the most mind-blowing for me, even just because of the scale. The intricate facade of the Duomo in Milano, the green and while marble exteriors of the Duomo in Firenze, the (neoclassical?) architecture of St Paul's, the unusualness of St Francis of Assisi, Notre Dame (haven't gone back after the fire yet), the bright golden interiors of pretty much any Orthodox church,... Hagia Sophia if I may stretch the definitions a bit :)