Nice, being the origional and "correct" version of the word, is most commonly used, however, the two other spellings are perfectly acceptable, though there are differences. Niice, more intense than nice, and yet not so intense as niiice, is often used when something is funny or cool, though not as cool as something that is awesome. Niiice, being the most intense, is only to be used when something exceeds the awesomeness of nice AND niice.
to admire or to really like. Sometimes used to pretend you like something but you are actually just trying to be kind.
btw the word niiiicce has to have 4 i's, 2 c's and 2 e's otherwise it has a completely different meaning. for example, niicccceee spelt like that means "wtf, you are a total idiot. i hate you so much! get away from me now. IF you dont comprehend wat im saying to you then ill kill you and bury ur body then water the soil on top of you and grow plants there".
As you can see there is a significant difference if you spell niiiiccee incorrectly.
niiiiccee! i mean ive seen better, but that's definitely top 5.
eg.
Sam: look at this new mobile
John: niiiiccee
Sam:your just saying that
John: nahh its niiiiccee
eg.2
Sam: look at this new mobile
John: niicccceee
Sam: oh you slack ass!
John: what do u mean?
LATER THAT NIGHT
Sam: i love watering plants.... especially over dead bodies!
muhahahhahaha....