To be objective about this is hard. But you really have to look at players that were the undisputed best at their position, then pick a few others. Probably no QB should be on the list, for example, because you might get 5 different answers from 5 different people of who was the best. But no will can argue with, say, Jerry Rice....unless they're extremely biased.
That said...
Jerry Rice
Reggie White
Lawrence Taylor
Jim Brown (stats and dominance that still put modern backs to shame when basically every other position has been surpassed in the modern era).
Based on stats alone, Barry Sanders is next on the RB list (and unlike most other great RBs, he played on a lot of crappy teams).
For their time, guys like Otto Graham or Unitas....but any great modern QB is better than they were from a purely athletic standpoint. And Montana is the most frequently cited "best QB" ever. Any of those are acceptable choices.
You might even add someone like Ray Lewis. Best LB in the modern era....probably ever, considering the advances in exercise and fitness technology that give the edge to more modern players.
Rod Woodson too, probably.