Let's have some examples. With Bach there are a host of possibilities. Consider the fugue: the competition is fierce, but it is all Bach competing with himself. This might be the perfect prelude and fugue, the C major from the Well-Tempered Clavier Bk 1:
But you could pick any number of other preludes and fugues, also by Bach. Then there are his concertos, cantatas, suites, sonatas and on and on. The truth is that most pieces by Bach are the most perfect examples of their form and genre. That's why he's number one!
So let us look at some other composers. There are several symphonies that each in their own way is an unimprovable masterpiece. Haydn offers us a lot of examples. How about the "Oxford" Symphony, so-called because it was played when he was awarded an honorary degree from Oxford? Back in the 18th century this really was a very great honor, not like nowadays when they hand them out like grocery coupons.
Or the "Jupiter" Symphony by Mozart:
And if anyone could follow those two masters, it would be Beethoven whose Symphony No. 5 is another unimprovable masterpiece:
There are so many possibilities, out of the last thousand years of Western music, but let me just pick a few. The motet, Nuper rosarum flores, of Guillaume Dufay is its own kind of unique perfection, synthesizing the older isorhythmic style with the new style of Renaissance counterpoint. It was commissioned for the dedication of the Florence cathedral:
As for the concerto form, we have unimprovable examples in the Baroque Era from Vivaldi, such as this, the Concerto for Two Violins in A Minor RV522:
Then we have perfection in the Classical Era from Mozart such as this Piano Concerto no 24 in C minor, K 491:
And in the Romantic Era with the Tchaikovsky Violin Concerto:
And the 20th century with the Sibelius Violin Concerto:
There, that should give you something to listen to over the weekend.