Sorry for the gimmicky and somewhat ambiguous question, but let's get to it.
It can be in any language (even a language you don't know) as long as you know what it means.
I really like Sehnsucht, it's just so precise (German has so many great, specific words like this one).
In the English language, I'm a fan of the ever popular flabbergasted. Debonair is another nice one. Leather kind of rolls off the tongue. I'll leave it at that before I end up adding dishwasher to the list.
It's a bit hard to choose one for Spanish, since I use it on a daily basis and it's normal to me. I'll think of something later.
In Japanese, I don't really know many words at all yet, but I really like that fireworks is written as 花火, which could be translated as fire flower.
EDIT: For Spanish, I'll have 'efímero' and 'borboteo'. I'd also add 'thump' to my favorite English words list.
