I love many of the English words listed already, but for some reason, there are two 'foreign' words that I particulary like - I think for the sound they make when speaking them outloud. I lived in Zurich for a few years where German is spoken, and the words "Ganz genau" really hit the spot with me. It means 'exactly right' or 'spot on!!' with emphasis! I also like the Hebrew word "Yofi", which means "Great" or "Wonderful".
Regarding Serendipity - I was doing volunteer English language teaching with a programme in Poland, and the group leader was a young man from New Zealand. Participants were given English words on a piece of paper and had to try and guess the meaning. One person had the word 'Serendipity' and this young man, told the lady to forget that, it was an old word that was never used, i.e. you would never hear it in modern conversation. I later took the student aside and corrected the information, and the meaning. She said "Oh, like it was serendipity that you heard him telling me that!" 