My take. There are probably 3 tiers of "first line" centers. You have your PPG elite centers such as Stamkos, Crosby, Getzlaf, Giroux, Malkin, Tavares, Kopitar, Sedin... should be pretty obvious.
1A centers - who can sometimes be elite but have up and downs. They can also contribute in other ways like how Toews and Datsyuk do on defense but somehow don't quite get the production the elite ones get.
Spezza, Thornton, Backstrom, Marleau, Toews, Datsyuk, Seguin
1B centers - are ideally your 2nd line center and on many teams, they are but sometimes your team doesn't have that star but these guys contribute. they'll get you 60 pts a season and they are your 1st line center because they are the best you've got. In their career, they might have one or two 1A-like seasons. The best 1Bs are consistent 1Bs or overachieving 1Bs. Could also include potential 1A centers who aren't reaching their potential.
O'Reilly, Stastny, Turris, Koivu, Richards, Stepan, Krejci, Desharnais, Kelser, Nugent-Hopkins