I would say that the most difficult variant to win is NL Hold'em because it is by far the most popular.
There was an article in one of the poker magazines that opined, if you want to win a bracelet at the WSOP, pick the tournaments with the fewest entrants.
If you look at the WSOP, all of the NLHE tourneys have huge fields and you have to have an incredible run of luck to just to cash, much less win. The fields for the lowest buy-in $1,500 NLHE tourneys have 3,000 players, and the only reason they aren't bigger is because they are capped.
I honestly think I'd have a better chance at winning the $50,000 HORSE tournament against 300 pros than the Main Event against 8,000 other players.
I suck at HORSE, but most of the games in the $50,000 HORSE tournament are fixed limit, so I could play super tight and hope for the best.
Against 8,000 other players in the Main Event, I'd have to be super-super-lucky just to cash, much less win.