When it comes to church art (and other interesting stuff) it's hard to beat the Basilica of Santa Croce in Florence. For quality and quantity, it's far more important than the Duomo (which is hardly decorated at all) and the other Florence “all star” San Lorenzo.
A short list of the highlights: altar frescoes by Taddeo Gaddi, frescoes by Giotto and Luca della Robbia, a crucifix by Cimabue (unfortunately damaged in the 1966 floods,) and sculptures by Donatello, to my mind the only sculptor who rivals Michelangelo in skill and emotion.
The crucifix of Cimabue deserves special mention because of its three dimensional effect of Christ twisted on the cross–a representation truly revolutionary when it was painted in the 13th Century. It was nearly destroyed in the Arno flood of 1966 to the point that it required almost complete restoration, which was only partially successful due to the extensive damage. The flood waters reached almost to the top of the work, washed away most of the paint, and warped the wooden cross so the point that most of the paint that had not been washed away flaked off the surface.
The “restored” version hangs in the church museum and still exhibits the extensive damage from the flood. Unfortunately, the light was so poor, I could not photograph it. A look at photos of the crucifix from before the flood will let you know just how much was lost.
Add to that the tombs of famous Florentines Michelangelo, Machiavelli, Galileo, and Ghiberti, and you've got a place you can walk around transfixed for a good hour or two.
The tomb of Michelangelo is of particular interest, not just because it's where the great sculptor is buried, but because the tomb was designed by, to my mind, the most mediocre artist who every littered Florence with his craft–Giorgio Vasari. Vasari, most justifiably famous for his book The Lives of the Artists, was also most famous for being a great salesman who convinced the patrons of Florence to hire him to do such works as this, and the interior decoration of the Palazzo Vecchio. The Vasari frescoes are particularly unfortunate when you consider that the originally commissioned decorators of the Palazzo were Michelangelo and Leonardo da Vinci, although neither of them completed their work.
To get the view over the roofs of Florence, you have to make two climbs. One up Giotto's Bell Tower next to the Duomo, and up the Duomo itself.
The former gives you great views of the dome of the Duomo (if that's not redundant.) The latter gives you views of the Bell Tower and, when you look out the other side, of Santa Croce–Florence's “Other” church.
What I like about this shot (other than the warm light near sunset) is it shows how huge Santa Croce is compared to its surroundings. It is, in fact, the largest Franciscan church in the world. It sports a relatively modern facade, done in the neo gothic style vaguely reminiscent of the design of the Duomo. It was designed by a Jewish architect in the 19th Century, and he worked a star of David design into the facade which you can see over the rosette window. Slightly incongruous in a Christian church.