I picked Laine because he's basically a younger Ovie.

Like others have said, preferably a guy that is a natural goal scorer and a guy that is good defensively and can go in the corners to get the puck out of the dirty areas.

For the goal scorer, you'd need someone that can keep up with McDavid's speed. First player I think of is Nikita Kucherov. Other guys include Tyler Seguin (technically a C, but has played RW in the past), David Pastrnak, and Johnny Gaudreau.

For the defensive/possession guy there's Brad Marchand, Brandon Saad, Michael Frolik, and Mark Stone.

I'd rank the line in this order:

Marchand-McDavid-Kucherov
Saad-McDavid-Pastrnak
Gaudreau-McDavid-Frolik
Stone-McDavid-Seguin (lower because Stone is on his off-side and is a bit slower, and Seguin is a natural C)

Marchand/McDavid/Pasta would be a crazy good line. Too bad Pasta isn't Canadian!!!