I think I found the best piece of music in Elsword... well the most calming...
Please note that this is the theme for the rest area in Altera a floating island, a heavily industrialized place. The rest area is nice and serene. Just a stretch of land at the bottom of the floating island, where a waterfall flows in the background. You can also see the mechanical "core" at the very bottom of the island in the background as well. (There is also a floating industrial billboard in the background that advertises the current event in-game. Other than that the Rest Area of Altera [which is called The Ponggo's Hideout] is a really nice place if you just want to relax and calm down. Especially when compared to the Nasod graveyard's outside the Rest area, and Altera itself which is industrialized.)