The heading of the thread is not "best I myself have ever seen." It's "All time." I have no objection to the former though with the number of younger posters, it would tilt the poll strongly in favour of the most recent stars. But if you actually respond to a post about "all time" first line players, the response can hardly avoid comparing across eras.
I voted on that basis for one of the two centres I never saw play, Syl Apps, but I won't repeat my post about him.
Just a note: I loved Dave Keon from his Calder winning first season. Voting for him makes a lot of sense and no criticism from me about that. It is a tribute to the Leafs of the sixties, however, that his line wasn't actually the equivalent of a present day first line. The first line would normally have been Mahovlich, Kelly and whoever played on their right wing, Bob Nevin in the early years. Keon's line played something more like the role of a present day third line. Of course, on differently structured teams, Keon might have been he first line centre, as indeed he was with the Leafs later in his career. And he was as effective in most ways as the first line centres, now Hall of Famers, he was matched against.