I start off with terrain. You know the drill pick and post.
Usually the concept, how the triggering will work. Finding the most efficient ways of getting what I want done. But that's all concept, so I put it in other.
When actually making, terrain.
i start off with terrain when i first open the editor, but i have a notebook where i write down all my ideas/triggering/story in
I make the locations and things first. Then the terrain, triggering, units, etc.
I like to know what I'm making before I go start, so I'd say ideas.
Terrain or Units/Stats
I chose Units/Stats.
Basic idea > a think over > Decide to produce or not > Start terrain > keep getting ideas and trigger plan > end terrain and about 70% of everything is planed > Produce triggers > finishing touches.
But I voted terrain because that the thing I start with when I start a new map in editor.

I think of the idea of the map, But when I actually create the map you have to from the bottom up and thats the terrain!
I come up with the idea first.
I do all the players, forces, unit names, etc... since I do campaigns its nice to get that hunk of work out of the way, and its usually the same unit names/stats/forces in all the missions, so I just have to memorize my characters and factions and that part's a breeze.
The usual Terrain! 
I usually do player setting so whenever I test the map doesn't look unproffesional/noob..then i do terrain

I don't see how people can start placing terrain before they know what they're doing.
Ideas or terrain. A lot of times I'm bored so I just open up the editor and make some random terrain and an idea comes to me. That's where Pie Tactics came from.
