やっば～。やばっ！ヤバい!　Informal word.You can say in both stuations of very bad and good.

Yeah, I'd pick ヤバい too lol I think it is similar to the word "crazy" in English. You can replace the word with すごい(extremely/awesome). Also, we often say like 超(ちょう)ヤバい when it is "super crazy" lol

ヘボい！ it's ugly.