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.
