The Uffizi Gallery occupies the pinnacle among art museums.
Not least because, having been built as the offices - uffizi - of the republic, the Florentines took the Greek word 'muses' and invented the word for a place for their exhibition: 'museum'. So this is the original museum.
And the building itself is a work of art, see the first photo below, of an external corridor.
One walks through an extraordinary building to see extraordinary things displayed in chronological order. Two pictures stand out in the field, for both of us, Botticelli's paintings LaNascita di Venere [Birth of Venus] and La Primavera [Spring]. The accompanying photos are from the public domain, see here, here and here. The history of the paintings is worth reading. Up close they distinguish themselves as large paintings with wonderful detail, delicacy and design.