The most profitable game is Omaha8. Both limit and pot limit. Both are populated by holdem players who have no clue how to play proper Omaha.
Read many articles on these two games and play them at the $50 level and you will make a steady profit that is several times as high as you could ever hope to make in Holdem.
Holdem is based on luck. You only get two cards and you only see 2/9 of the cards you will use to make your hand. The biggest edge in holdem between AA and 72o is about the same edge you have on many omaha starting hands against many others. AA23, AA24, A234, etc, all blow out of the water hands like 45TT or 78JQ. You rarely get huge edges like that in holdem, and bad omaha players want to play every single hand because they think any four cards could make you a winner.
The key in limit Omaha 8 is playing proper starting hands. Understand what is a good starting hand and raise it before the flop, fold everything else.
The key to PLO8 is freerolling and quartering. You always get people calling their entire stack on a naked nut low and you hold a good high and the nut low...you take half their stack. You also get people who flop the nuts and call off their whole stack hoping for a blank on the turn and a blank on the river when you hold the same hand they have plus a draw to a hand that can beat them.
Learn these concepts and you will win money. The problem is that playing Omaha is boring as hell. There is hardly any bluffing
and you fold a hell of a lot more on the flop than you do in holdem.