Nicest is like the best and most "nice" thing or person you can get. If someone is the nicest, they are the most kind and loving person in the world. If something is the nicest, it is the best and the most satisfactory thing you can find. Its a very simple term that can mean a whole lot.
Girl 1: Dayum girl! Those jeans are the nicest!
Girl 2: Totally! I got them on sale at (insert great brand here)! So comfy and stylish!
***OR***
Guy 1: Yo man. Did you see her! Abby is the nicest person in the world!
Guy 2: Yea bro. She totally is the best!
A guy who cares and is kind to everyone, may have had problems with being a faggot in the past but became really nice which makes him even nicer, common names of nicest guys ever are Thoren, Nathan and danny
Person 1: he's was such a faggot Person 2: no Thoren is so nice now Person1: he must the nicest guy ever