Tonelli wins this. He assisted on Nystrom's goal to start the dynasty. He scored the two big goals in 1982 vs. Pittsburgh (the tying and overtime goal) in the only series the Isles were in danger of losing. Then he sort of made the play that upended a Rangers player who lost the puck to Sutter who shot it and had Morrow slap in the rebound in 1984 in overtime. He was there for a lot of key moments. If you were to ask me who deserves the HHOF more, Lowe or Tonelli, I'd have said Tonelli every time, and still do. Lowe is the one who got in though, but he didn't have Tonelli's impact. I don't think Tonelli was elite enough for long enough to be in the HHOF either, but he was better.
I will say this, Lowe was more important in 1988 and 1990 when Coffey was gone. But it was more of a committee on defense at that time and 1990 had plenty of players stepping up, not just him, to make up for Gretzky being gone.