Based on my personal experience, I would actually say suspense is one of the hardest genres to write! It requires a knowledge of pacing, whether you're building that slow suspense or writing an action-packed wild ride that never stops. It also takes a lot of creativity to write your characters into holes and find ways for them to get out again, as well as to keep your readers guessing. And finally, I think writing a multi-dimensional villain that people can both believe and relate to is tougher than writing a hero who's that way. I love reading suspense, but I have always struggled to write it well.
Based on perception, I would also say sci-fi and fantasy are hard to write because, like Mare said, you often end up creating different worlds or at least changing the rules of our world, so it requires a great deal of consistency and attention to detail. I haven't written enough sci-fi/fantasy to really know firsthand though.
For me, the easiest genre has always been drama. The storylines tend to be simpler and slower-paced, and there usually aren't as many characters outside of the Boys to juggle with. The focus is less on action and more on characterizati on. I feel that romance is similar to drama in that respect, although romance has the added challenge of trying to do a unique take on a universal storyline. That said, it takes a strong character writer to do drama and/or romance well. So I think the question of which genres are easiest and hardest really comes down to what your strengths as a writer are.
