so then you think there's at least 64 centers who are better than him at 5v5, care to show a list of those?
Well firstly I wouldn't say there are 64 true top 6 C's in the league. Probably closer to 45-50.
At 5v5, I'd take these guys over Monahan, in no order:
Crosby
McDavid
Malkin
Matthews
Eichel
Barkov
MacKinnon
Getzlaf
Aho
Larkin
Couturier
Backstrom
Kuznetsov
Barzal
Tavares
E Staal
J Staal
Keller
Petterson
Bergeron
Horvat
Seguin
O'Reilly
Point
Stamkos
Kadri
Scheifele
Nylander
Krejci
Pavelski
Couture
Turris
Duchene
Karlsson
Johansen
Draisaitl
Toews
Kopitar
Schenn
Stepan
Backlund
Trochek
Drouin
Lindholm
Hischier
Hayes
RNH
Dubois
Little
Johnson
Perreault
Zibanejad
Domi
Kerfoot
Schmaltz
I'd say Monahan at 5v5 is a high end 3C low end 2C. But again, he's much on the PP than a ton of the guys I've listed above because he's such a great finisher in the low slot - so overall I'd say he's a 2C. But in the other 95% of the ice he doesn't excel - he's a very poor puck carrier which really hurts the breakout, and is too slow to be effective defensively, especially off the cycle.
Look at some of the plays Tavares has made to set up Marner - you'll almost never see that from Monahan.
I'll finish by reiterating that Monahan is very much a product of his linemates - but that can be a good thing at times. If you play him with 3rd liners, he'll produce like a 3rd liner, if you play him with 2nd liners, he'll produce like a second liner. If you play Tavares with 3rd liners, he'll produce like a 1st liner.
I think the ideal spot for a guy like Monahan is as a top 6 winger - remove some of puck carrying & defensive responsibility.