Absolute playing strength is not an easy term to define. But it's mainly a question about the objective strength of moves played, and can to some extent be measured by today's strongest chess players - the PC chess engines. It would have been very interesting to magically put together chess players from different eras to fight it out over the chess board. That will never happen and that's exactly why it's such a great discussion subject.
My preliminary nomination for the top ten players of all times, based on actual peak playing strength is as follows:
No doubt many players of the older generations would quickly adopt to the new situation and raise their level of play. It would not take them too long to catch up on the opening theory in their repertoire and - possibly more important, simply to realize that chess is a professional activity, requiring their full attention. That could even happen during the relatively short duration of a tournament or a match. Still I believe today's professionals, battle hardened from countless tournament games since their early youth and grown up with huge databases and strong analysis engines would come out on top.