McDavid
I'll take the best chance at the best player of this generation since Crosby.
McDavid vs Eichel
They're in a tier above Hannafin, but if Eichel is a bacon and 3 cheese macaroni casserole then McDavid is the delicious slightly burnt and crispy corners that everyone fights over. To put it another way, I think the next decade will have a handful of Eichels (3 or 4), but if we are all really lucky there might be 1 other McDavid. There's Crosby and there's the rest of the league.
The need for a top line winger
McDavid turns any top 6 winger he plays with into a top 3 winger.
The need for elite level forward depth
Agree, would be great to have another top 30 player in the top 6. What I really want is to have him on a different line driving a true multi-line threat. What I don't agree with is that Hall + Reinhart pushes that multi-line threat to anyplace particularly special compared to Reinhart + something else. Hall's game to game impact doesn't impress me in a way that I think, this type of player is not attainable in any other way. If you want a player like Hall, I think we can get him with either a 2016 1st at the draft, or with the 2016 1st in a trade.
McDavid vs Eichel + Hall
The value is there and I don't think it's an arguable point. The arguable point is whether it's worth giving up the best player for that value. What's the value of knowing that on any given night you have the best player on the ice? From a selfish perspective, what's the value of having that player as a Buffalonian? Even if he doesn't become the next Jim Kelly he will be a Sabre and the Sabres are Buffalo's so by extension I get to strut that around every airport I fly through.
The team
If I'm trading McDavid, the only check box on my list is "does this trade take the team to a level that they cannot achieve in any other way?" I don't believe it does. I believe that getting a Hall type player is not something that we won't be able to do in any other way. Following the 2015 draft we will have 2 seasons that, up until it's demonstrated that the Sabres won't be drafting top 5, will have 1st rounders with immense value. With the new draft rules they will probably both be lotto tickets of more value then the pick we got for Vanek. There's enough depth in the prospect pool that combined with those picks we can find our way into another top 6 forward. Heck if Edm fails to get in on the McEichel dealings we may be able snag Hall anyway.