Crosby line quite easily.
I don't like Ov/Malkin. They've never had success together. Both insane players, but on a line? This excites me the least (and I think Kucherov is awesome).
Matthews/McDavid/Marner obviously could work great....not sure off hand what chemistry would look like. Maybe it'd be awesome, maybe not.
Mack/Ranta already have insane chemistry. Best in league (certainly best in this poll, as they're the only 2 linemates). Sidney Crosby has the highest hockey IQ out of anyone here. I think he'd find a way to mesh with those guys.
So if the concept is that all 3 players have to play together on a line, i'm thinking Crosby first, McDavid 2nd, Russians last.
If the question was what trio you take without having to play them together on one same line - it becomes a lot more interesting.