So which one do you think is easiest? I pick mini-game because the map can be the way you want it. For your rules. Not someone elses. Feel free to discuss.
Crappy minigames are easy. Good and inventive minigames are very hard. Crappy competitive maps are far too easy. Good competitive maps are somewhat difficult. Inventive and good competitive maps are very hard. Any form of aesthetic map is easy, it just takes time. Puzzles are by far the most difficult, since a good puzzle requires you to be inventive.
So, basically, crappy maps are easy to make. Good maps are hard to make, regardless of what gametype they are for. That's pretty much how I feel, as well. You can't really judge how easy any one genre of map is to create. Each map can be made very thorough and complex. It just depends on how well you want to make a map. That being said, I would say that it's easiest to make an average infection map or racetrack.
I agree with Rorak, but beyond map each map maker is good at different types of maps. In my opinion, I am pretty good with BTB 1 sided objective, and that is my area of interest. My 4 v. 4 and 2 v. 2 maps tend to be okay, but not as good as others. If I tried to make a mini game it would probably be horrible, and any race track or infection map I make will likely be sub par. Why is this? I love BTB and one sided objective, or really just objective in general, so that is what I focus on and try to learn about. A good map is one you work hard on, and the harder you work and the more maps you have gone through learning all you can about that type, the better your maps in this category will be. The reason it is easy to make an average infection map is because most bring that average pretty far down.
Easy to get an idea for =/=easy to make. I could come up with Invasion ideas all day long, but I'll be darned that I STILL can't get spawns to work