I think there's nothing better in the world than love. A+다고 생각하다 This is a grammar pattern meaning "to think that (adjective)". ex. 저는 고양이가 진짜 귀엽다고 생각해요. I think that cats are really cute.

I think that anything isn't more important thing than Love in the world. 없다 is opposite to 있다, 있다 means is 'be verb'(including meaning of exist) 고 is like that, but In Korean, using '고' is better than not using.