Here's how I see it. Certain games are harder not because of the knowledge needed, but because of the cue ball control needed.....wait Ghost and all you other 1holers!

....keep reading.
9ball (the game I learned first) is easiest IMO because you can have a loose cue ball and still get "area" position and run out. Safeties can also not be perfect, but work none the less.
14.1 (I learned 2nd) is next hardest IMO because the cue ball needs to be on a short leash. Yes the shots are mostly short, but you better be able to stop the cue ball on the correct side of the shot! MOST good players can learn 14.1 in a relatively short period of time from A GREAT 14.1 PLAYER. An excellent teacher will get you up to speed if you can already pocket balls well.
1 Pocket (I learned last) is the hardest for most because there are times when you need to set the cue ball EXACTLY on a spot or you sell out. Also the more creative/abstract your mind the easier the game is. This game also requires you to go from "safety" mode to "fire away" mode from 1 shot to the next which can be tough. Kind of like getting hit with a big number at 14.1 then having to get up and run balls! There are all aspects of every other game in 1hole....banks, patterns, billiards, shot making, safeties, kicks......no wonder Efren rules that world huh!