The ne plus ultra of sad pieces in major keys in my mind is the Chopin Etude Op.10 No.3 in E major, which is the only piece I can name offhand which has moved me to tears. There's something about slow pieces in major keys with four or more accidentals which makes them sound somewhat melancholy to my ears, so I'd also tentatively name the slow movement of Chopin's B minor sonata (the coda of the Liszt B minor sonata I'd be more inclined to label tranquil than sad) and possibly the slow movement of Beethoven's 'Emperor' concerto, both of which are in B major, as well as the infamous Largo from Dvorak's Symphony No.9 and possibly the slow movement of the Grieg piano concerto, both of which are in D-flat major. Scaling back the number of accidentals, there's something of a melancholy edge to both Brahms' Intermezzo Op.117 No.1 in E-flat major and Rachmaninov's Prelude Op.23 No.6 in the same key.
As for 'happy' minor key pieces, I'd add from WTC Book I the C minor prelude and fugue, D minor prelude, E minor fugue, F-sharp minor prelude, and A minor prelude and possibly fugue. Most upbeat minor key pieces, though, sound rather more angry or agitated than 'happy' to my ears - an argument could perhaps be made for some of the faster Chopin minor key etudes (Op.25 Nos.4 and 5 leap to mind), but then again perhaps they are too frantic to be happy.
At least, this is how I hear these pieces. Again, some people might think of the major key pieces as placid rather than sad, or of the minor key pieces as angry rather than happy. (There's a reason, I'm sure, why in Nielsen's Symphony No.2, 'The Four Temperaments', the 'choleric' movement is fast and in minor, the 'phlegmatic' movement is moderate and in major, the 'melancholy' movement is slow and in minor, and the 'sanguine' movement is fast and in major.)
Topic: The most beautiful and saddest music in MAJOR KEYS