Today's the 150th anniversary of the birth of Carl Nielsen, which must be why his bio's the featured article on Wikipedia today. Nielsen is the best-known composer from Denmark, a fact of some annoyance to other composers from Denmark, but he's still not all that well-known internationally. Together with the better-known Jean Sibelius from Finland, just 6 months younger, Nielsen is to me one of the great symphonists of the generation following Brahms.
I first encountered Nielsen's music in the form of a student orchestra performance of his Fourth Symphony. I didn't know the work well enough to judge, but it was probably a terrible performance. Certainly I started putting Nielsen on my least-favorite composer list (which actually existed: the Schwann catalog ran a reader survey to that effect). But I kept seeing Nielsen discussed as an important figure in the history of the symphony, a source that had given me many favorite composers, and conscientiousness nagged me that I shouldn't leave it at that.
Fortunately, what I picked up at random was his Second. The Garaguly/Tivoli recording, which I heard, doesn't seem to be online, but here's the excellent Blomstedt/SFS recording, which did not then exist. This work is titled "The Four Temperaments" and ingeniously maps four types of human character onto the traditional four movements of a symphony. I loved the whole thing, but particularly the mostly-violent "Choleric" opening movement. The climax of the development (3:45-4:30 in this file) immediately struck me as, and remains for me, one of the most thrilling moments in all classical music. (The YouTube notes don't say, so I'll add that the remaining movements are, in order, phlegmatic, melancholic, and sanguine - though you'll hear that each mood contains hints of the others, since no real human is a pure example of any of these.)
What most struck me about Nielsen's early music was the sheer bounding physical energy of it. I have a sort of private pantheon in which Bruckner is the symphonist of the soul, Shostakovich is the symphonist of the mind, and Nielsen is the symphonist of the body. In the First, you can hear this deriving from the more energetic side of Dvořák, and the Second and Third build on that model with increasing maturity. I still think the Second is my favorite, though recently I've found myself leaning towards the First, whose candidate for most exciting moment comes with the 16 repetitions of a forte "snap" in the scherzo (17:26, repeated at 19:18).
But that's just his early music, and just the First through Third Symphonies and a few of the smaller orchestral works. His chamber music, even when written in the same idiom, doesn't come out that way, and his later music, including the Fourth through Sixth Symphonies, evolve a quite different idiom. Most accounts of Nielsen claim his Fifth as his greatest symphony. It's certainly a strange, striking, and original modernist work, but I can't claim it as dear to my heart as the early works.
Denmark avoided involvement in World War I, but, judged by chronology, it seems to have given Nielsen a severe case of PTSD. His Fourth, written during the war, is disintegrative: little elements of his earlier style keep getting undercut. I find it a very unsettling work. The Fifth, unforgettably manic-depressive, is very striking and more satisfying, and something of an ancestral model for the later Swede Allan Pettersson. The Sixth is the oddest, querulous and enigmatic, and my favorite of the latter works. Here's a concert recording of it, and listen for the passage at 7:57 where the music suffers the composer's then-recent heart attack. (Shades of Terry Gilliam and the Black Beast of Aaaargh!)