This was a toss between the 'funny thread' and this one, but it is cultural so here it goes. A language containing thousands more nouns than English but yet so... redundant in daily conversation between friends (and it completely gets through, pronoun-less and all)
I can confirm a top 5 I hear day in, day out at school from my classmates...
やめて、恥ずかしい (Stop it, it's embarrassing)
やば、やばくね、こわっ (yaba, yabakune, kowa)
ねねねね、(Hey, hey hey)
うそ！(No way)
明日ダイエット (Gonna start dieting tomorrow, well maybe not top 5 but close, anything to do with weight is daily)
And others...
なんで、こんな美味しいの？ Why is this so delicious?
(I'm -never- sure how to respond to this in Japanese as sarcasm only comes to mind. If one cannot play her role well, then just nodding in agreement will get me very far)
びっくりした！(bkikkurishita = You scared me!)
This a common one for all Japanese, I've had to make myself say it whenever I'm actually surprised, as I noticed in English we don't have a 'one set' expression, we respond with various (curse) words or sounds.
I am surprised that 'kawaii' was not in there, that's a major minus point from me as if I had a quid for every time I heard that every day at school, I'd be swimming in money...