For me, the easiest is drama with a hint of romance. My dramas tend to focus mainly on one or two characters, instead of the whole group and more, which helps to simplify the storylines. I think it's easier when you have fewer characters and subplots or interwoven storylines to keep track of. Also, drama/romance seems to be more straight-forward, not as many twists and turns in the storyline. It's easy to plot.
That said, the hardest genre I've written in enough to know is suspense. I love suspense, but writing it is a challenge. My suspense stories often have multiple storylines woven together, involving a variety of characters I have to keep track of, and I usually try to throw some twists in there to keep people guessing. It's hard for me to avoid it being predictable and to avoid plot holes, because it has to come together seamlessly. I am an outliner, which works well for drama/romance, but doesn't always lend itself to suspense. When you're trying to keep people guessing, you don't want to get set in one direction; you want the freedom to be able to change it up. I am not a fly-by-the-seat-of-my-pants writer, though; that's how I get writer's block, when I don't know what comes next. That's why suspense is a fun challenge for me.