Most difficult game in which to be a skilled player?

[dibbs]

Was just debating with a friend about this, did a search but couldn't find anything, I have a feeling this may have been discussed before however. So, which poker game is the most difficult to become a skilled player in?

LHE, NLHE, O/8, PLO, Stud, Stud8, Triple Draw, 5 card draw?

I realize the question is quite vague and there is no definitive answer, as each game caters to somewhat different skills, but just curious to see what people think.