Nowadays, you can't confidently sacrifice material like Paul Morphy did without calculating every single possibility
I disagree.
Make sure you get someone with your style though. Either Jeremy Silman or Yasser Seirawan (both are prominent chess authors, so I get their literature confused sometimes) said that they started studying the games of Alexander Alekhine, but their style was completely the opposite of Alekhine's. I think Seirawan said that. Hm... Either way.
Actually for a lot of Silman's early career his style was not at all completely the opposite to Alekhine's.
I study the world champions and their opponents.
I own a plethora of Bobby Fischer games, including his work, "My Sixty Memorable Games," Garry Kasparov's, "On My Great Predecessors Vol.IV - Fischer," and Lou Hays' "Bobby Fischer - Complete Games of the American World Chess Champion."
I also Study Tigran Petrosian, Garry Kasparov, Aron Nimzowitsch, Miguel Najdorf, Samuel Reshevsky, and all the great ones. http://www.chessgames.com/ has almost all of the great games by the great players, so you should check it out. I go there so much that I decided to just make it my homepage. If they've got a GM or IM title, then you can't really go wrong with studying them.
Older ones like Morphy and Alekhine are great as far as tactics go, but the openings are obsolete, and chess theory has evolved since then. Nowadays, you can't confidently sacrifice material like Paul Morphy did without calculating every single possibility, so that's the only real problem with pre-1900 games.
All of the world champions are good chess players (hence the world champion title) so you can study any of them. Make sure you get someone with your style though. Either Jeremy Silman or Yasser Seirawan (both are prominent chess authors, so I get their literature confused sometimes) said that they started studying the games of Alexander Alekhine, but their style was completely the opposite of Alekhine's. I think Seirawan said that. Hm... Either way.
Kasparov, Fischer, and Karpov are generally thought of as the three best chess players ever, so that's where I would start if I were you.