cmus offers a great set of features including the essential gapless playback. It’s extremely frugal with system resources. The software starts fast, offers a very clean interface, and requires few dependencies.
cmus is respectful to your filesystem. It doesn’t automatically download album art, or create any hidden folders in your music collection’s directories.
Another strength of cmus is its easy to use filters. You have real flexibility with its search parameters.
Unlike many music players, cmus is well-documented. There’s a good cmus tutorial which explains the basics.
cmus runs musikcube very close. Both warrant a very strong recommendation.