I don't know about the hardest or easiest, but a hard one to mention would be Real Time Strategy.
Programming is not simple, because you have to manage AI and path-finding of sometimes thousands of instances. But the true challenge is balancing it for online play when you have to try to account for every single possible combination of units and strategies to work against every other one. Especially if you use multiple playable teams that play uniquely different.
Yes. I am very fond of Starcraft II and Brood War, but even SC2 is like, 10 years old now. They just remastered Brood War with HD graphics though, which is pretty awesome. Same terrible pathfinding and unit control though. But it's one of my favorite games so I'm actually glad it's still the same really.
You mean with the enemy path finding? Yeah it can be easy but I think part of the challenge is making it look like the enemies aren't just following A* algorithm paths or whatever, plus custom behavior. I will say it is much easier to design any genre today than it was 10 years ago, that's for sure.
