As a ten year almost full-time poker player then I have often been asked which is the most demanding form of poker. For me there is only one and that is NLHE when played with a deep stack. It just adds so much complexity to the game that many novice and even intermediate players never really figure the game out.
It combines bookmaking skills as well because you will be constantly taking and laying odds all the time in no limit. These days I play on sites like 888 poker for the volume of action and I am amazed at how many mistakes I see even at levels as high as NL100 which is $0.50-$1.00.
Also there is one other very important factor as well and that is to do with the fact that HE is a two card game. While you may be wondering what on earth I am talking about here then let me explain. The fact that NLHE is only played with two starting cards instead of four like in Omaha means that hitting the flop is much more difficult. All of the players know this and so each player then knows that the other is unlikely to have connected. So it then becomes much easier to launch big bluffs and re-bluffs in this form of poker unlike in say limit or PLO.
This fact of the game can make it very difficult for some aggressive types once they reach a certain level of sophistication. I have known many players who simply spewed buy-in after buy-in away by being too aggressive in the wrong set of circumstances. One slight moment of indiscipline is really all it takes in NLHE and your stack is history.
So for me then there is really no comparison. I have played every form of poker known to man but NLHE is the most difficult to master but it also can be the most financially rewarding as well.