Whats the best tools for making a new SoC level? l

[ket]

Thats right guys.. I'm hitting SoC hard and was wondering what the best tools are for importing existing MP and SP level content and for creating level content. My first objective is to give the mercs not just a base instead of that shitty little outpost at the military warehouses, but a whole level for them. Yes there will be missions for the player to carry out on this level etc, but for now I need to get the level built So all suggestions / ideas / general input and thoughts are welcome!