Santa Maria del Carmine - The Brancacci Chapel, nuf said.
San Miniato del Monte - sits on a hill in Oltrarno, the other side of the Arno river. The story goes, St. Minias was beheaded on the banks of the Arno during anti-Christian purges. After the deed was done, he picked up his head and walked to the spot where the church today stands. I hiked up to this church, with my head on my shoulders, on one of my last days in Florence. I arrived in time to hear the monks chanting. Is it sacreligious to say it was a magical moment, because that's what it was.
Siena Duomo - My guidebook described the Siena Duomo as, "either a symphony in black and white marble or a tasteless iced cake." Ouch. I sort of liked it because, let's face it, I grew up attending a church made from cinder blocks. Not one inch of the exterior or interior of the Siena Duomo is without engravings, mosaics, frescos, marble, gold, etc. It also has quite an array of depictions of the slaughter of the innocents which I found very disturbing.
San Marco, Venice - I stood in line for an hour to see the inside of San Marco. It was well worth the wait to see the interior which is full of gold and marble mosaics! One sample is below, and below that is my cohort, sans John who was snapping the picture, standing in 'St. Mark's Square'.