The Tempietto, Donato d'Angelo Bramante, in the Courtyard of St. Pietro in Montorio, on the Janiculum  in Rome, 1502 ff.

This is one of my favorite buildings of all time.  It's little, and a little out of the way, up a short flight of stairs from the street on the Janiculum, one of the hills which rise to startling vantages on the outskirts of the core of the ancient city and the Vatican.  I first heard of it, and saw it in the book of an undergraduate mentor, the architectural historian, Bates Lowry.  I was enthralled by the building and by his analysis.  I'd never heard of architecture spoken of in this way:  

It's a centrally planned building, a radically simplified and spare fever dream of Bramante's proposal for the evolving rebuilding of the new St. Peter's around the fourth century St. Peter's Basicilica in the Vatican. In plan it's a perfect circle inside a larger perfect circular Doric peristyle (colonnade) capped by a balustrade (row of balusters--like elongated, inverted amphorae).  In elevation (the face--or façade of the building) it's a cylinder of columns, screening a taller internal cylinder capped by a dome.  The circle was a desideratum of particularly church architecture in the high renaissance--the years around 1500 in Italy, say.

The Tempietto's function is a commemorative mausoleum, an ancient form for tombs, with its focus on the middle. It stands in t cramped, undistinguished courtyard beside the church.  I first saw it in person with Yvonne Mazurek, the art historian from SYA in Viterbo (a little over an hour north of Rome), and other friends, when I was speaking there.  I've seen it and photographed it a couple of times since (though my own photographs, are presently unavailable--long story, one I'm working very hard on).

Perhaps even because of it's small size, it was able to be built readily of load-bearing stone masonry--one of the purist expressions of an "ideal" renaissance architecture vision.