I've put the most hours and work into NLHE, for obvious reasons. I do really love the game.
My other favorite is 2-7 no limit single draw. Just 2 rounds of betting (pre and post draw). The information set is so different from NLHE, and that along makes it interesting. There are a couple of
odds things I learned a few years ago that I don't think most people realize, and that become very exploitable at times. I don't like 2-7 Limit triple draw nearly as much.
I've always enjoyed 7-card Stud, and have warmed up to Stud8 and even RAZZ.
PLO is interesting but drives me batty. I hate how close the equities are at every moment.
All limit games. But most of all I like 7 card stud high-low and Omaha high-low.
I much prefer Omaha high-low 8 or better to PLO.