im not sure HORSE qualifies as an answer because it is 5 different games
in all games I say draw games are the hardest, or maybe chinese poker
but to the question - the skill sets are different to some degree and the same in others, so its hard to compare - however I think omaha holdem is trickier than texas
holdem because you have to understand draw potential and bet sizing much much more.
least amount of luck, well if you go by variance (lower variance = least luck) then omaha is less luck than texas, and smaller tourneys and smaller cash games are less luck than bigger
sng's are probably the easiest because they pay deeper
cash vs. tourney - cant really compare because effective play styles can be drastically different, and it really depends on what you want out of it, but you will lose less in a tourney because often you can not buy back in, and if you can its limited