For real concentration I can't have lyrics but that's a great idea for other flow states. Mozart and Brahms are good for me ... Not slow enough to put me to sleep not fast enough or unusual to make me pay attention to the music.
I vary a lot but when I do classical music Mozart has occupied quite a lot of my stats, in particular a clarinet concerto by Katherine Lucy [1] and also things like Beethoven's 6th (pastoral, it's beautifully featured in Fantasia) or Grieg's morning mood.
