The most difficult? The ones I'm looking for.
Haha, seriously though, I think of you're looking for a specific player, you could be at it for awhile.
Some people that I've talked to believe that some teams are more difficult than others. However, if you adjust the population reports for the number of players represented on each team, the only difference is that some teams have more players (and thus have a higher population of cards in total).
I also believe that a large number of T211s are in raw collections and that the population reports can only be used as a guide.
Just my two cents.
Cheers,
Steve