I'll give it a shot (in no particular order):
Crosby
McDavid
Backstrom
Scheifele
Seguin
Eichel
Bergeron
Stamkos
Getzlaf
Kopitar
Barkov
Tavares
Matthews
Malkin
Giroux
Pavelski (when he plays C, anyway)
Kuznetsov
I'd say the above list is all guys who are inarguably better than Monahan, and there's a small list of guys who I think could be argued one way or the other:
O'Reilly
Johansen
Couturier (depending whether he can repeat this level of production in the future)
Trochek
MacKinnon (who is currently the most overrated player in the NHL)
Barzal (too soon to call it)
Toews
Draisaitl (when he actually plays C)
Contentious possibilities:
Backlund and Kesler (depending how much you value offensive production over defensive ability)
I may have missed a couple of dudes, but I'd say Monahan is firmly in the top 30 centres in the league, even if you value defensive forwards highly, and he's most likely in the 15-20 range.
I agree with your first list although think Pavelski and Kuznetsov make it just barely. I would put MacKinnon there too, what he has done this year is super impressive and although he might not always be at that level I think he will be close.
Out of that list I would say that the only guys even remotely possible to be traded for (or acquired) are Tavares, Backstrom, Kuznetsov and Pavelski. I think the chances are very slim that we could get any of them but the other guys are all definite no's.
I think of the other list Barzal, Draisaitl (unless some multi-team deal where someone else deals for him and then flips him to us which seems highly unlikely) and Johansen are definite no's as well.
I would disagree with you contentious possibilities and just say flat out they are not better or even close to better so I will eliminate them.
So that leaves a list of: Tavares, Backstrom, Kuznetsov, Pavelski, O'reilly, Couturier, Trochek, Toews as guys who are possible upgrades on Monahan and realistic for us to add. For the purposes of this the point is getting a #1 center that can we can win the Cup with. I don't think they are better than Monahan anyways but I can definitely say they do not fit into the elite/win a cup with center category so I will take out O'reilly, Couturier, Trochek, Toews (Toews of this age not prime Toews).
So that basically has us down to realistic options to replace Monahan and have a chance to lead us to a Cup as Tavares, Backstrom, Kuznetsov, Pavelski and even at that I am not sure any of those outside of Tavares really fulfill the 2nd requirement and saying the latter 3 are realistically possible to get is a stretch as well.
Some may disagree and I would love to hear others (especially those that think we should move Monahan to upgrade) but I think I was fairly reasonable in my breakdown of guys.
I will throw one other possibility that I think most Monahan isn't good enough guys won't like. perhaps you think a guy like Nolan Patrick may develop into the better C, well does a deal with Philly around Gaudreau, with more pieces than Patrick I am thinking something around guys like Konency and/or Provorv, get us the center we want? It does mean losing Gaudreau though so may defeat the purpose in some folks eyes.
TL;dr- There are very few guys (Tavares, Backstrom, Kuznetsov, Pavelski) that are realistically possible to get and upgrades at Center to Monahan. For those that think we need that upgrade and propose doing it I would love to hear how we get one of those guys or a guy that you think I omitted.