Noun, meaning any pleasant gesture resulting in a favorable outcome. By not being a nice one, it means that the person being talked about is, in fact, not a nice one. Although this is a noun, the context may vary and can be used as a pronoun, adjective, and about every other form you can think of.
'Hey, Todd, can you hand me a nice one?'
'Scott, you are not a nice one.'
, I just wanna know why this is such a nice one.'
'Yea, everyone thought about havin sex with her while she was blacked out'