Who Were the Hardest Players to Beat?

Here's an interesting perspective on the question.

If we take the list from the original post (#1) and remove all the active players, we're left with a list of the toughest non-active Grandmasters ever to play the game.

Given this career perspective, it becomes apparent how the cream rises to the top, where 12 of the top 15 toughest players to beat were at one time World Chess Champions! And of the remaining three, Paul Keres finished 3rd in the 1948 FIDE World Championship Tournament, behind Botvinnik and Smyslov (both world champions), Ulf Andersson was once the 4th highest rated player in the world, and Isaac Boleslavsky won the 1951 Candidates  Tournament, tied with David Bronstein, but lost to him in the 2nd tie-break by a single game (+3-2=9). Bronstein went on to tie Botvinnik in the World Championship Match, with Botvinnik retaining his title (you have to beat the world champion to become the world champion).

Given the thousands of players in the recorded history of chess (where statistics are available), what are the odds that these results were purely coincidental? It's no surprise that the stronger the player the harder they are to beat.

I think Petar Trifunovic was renowned for being hard to beat. He gave many players headache.

Very true. He was nicknamed the Drawing Master because it was so hard to win against him. He lost 8.83% of his games, so he should be number 4 on the list in post #43, ahead of Karpov.