Kirei and Kirai....because its really funny to hear people new to Japanese mix the two up at a bar...good times.
other than that, I like jirojiromiru (to stare at)...just cause its fun...oh and tokidoki is fun too...and also used for like 90 different things including the sound of the human heart.
I love the way japanese girls say "Sugooooooi!" or "Kawaiiiii!"
One of the first words I learnt in japanese was "しんじまえ" (shinjimae) wich means more or less "Go to hell"
And I really like "なにあねん" (nanianen?) What the hell? and "ゆったとりでっしょ" (yuttatoridessho) I told you so!