Whitehat1963: It's on the Guess-the-Move page's "special functions" section. For the moment, here's the top 20 hardest:1. Maroczy v. M. Morgan, 1906 (recent entry, still only 9 plays)
2. Tarrash V. Gunsberg, 1890 (been on here for quite a while, lots of plays)
3. Siciliaans v. A. M. Giustolisi, 1955 (a recent entry, few plays)
4. Keres v. A Fuderer, 1955 (recent entry)
5. Koltanowski v. NN, 1953 (recent entry)
6. S. Bogner v. K Szabo, 2007
7. S. Fedorchuk v. Petre Nad-Titus, 2011 (recent entry)
8. J. N. Sugden v. Keene, 1965 (recent entry)
9. Leko v. L. Bruzon, 2005 (recent entry)
10. Judit Polgar v. Mamedyarov, 2002 (this one's been on here for quite a while)
11. Anand v. Van Wely, 2001 (recent entry)
12. M. Fiorito v. Krantz, 1988 (recent entry)
13. H. Janssen v. Euwe, 1979 (recent entry)
14. Nezhmetdinov v. P. Ermolin, 1946 (been on here a long time)
15. Lahno v. M. Seps, 2004 (been on here a long time)
16. Ivanchuk v. Anand, 1988 (been on here a while)
17. T. L. Petrosian v. Le TD, 1994 (recent entry)
18. Shirov v. Kasparov, 2002
19. Blackburne v. Gunsberg, 1890 (recent entry)
20. Psakhis v. P. H. Nielsen, 1994