I grew up in NY and lived in NYC and I still call it a plain slice.

I've always called it a plain slice but I've heard regular/cheese/plain used interchangeably

I'm from NYC and call it "plain" or "cheese." "Regular" makes me think too hard as to whether the "New York thing" is to say "regular" or "plain." It's also a lot of syllables.

A similar phenomenon occurs with waiting "in [a/the] line" vs "on [a/the] line." I think I say either, but I'm "supposed to" say "on line."

when you order, it's called "a slice".