The most efficient way is definitely to use an existing editor.
Use an existing level editor - or maybe even just a pixelmap editor or text editor - and then write whatever conversion utilities are needed to produce the data in the format you want.
Some editors already have some mechanism to make plugins - for example, using a high-level language that won't waste your time too much.
Also consider the amount of time it's going to take your content team to build maps.
A feature-rich third party editor with plugins, add-ons, and conversion utilities, is likely to provide a better user experience with less effort, than trying to roll one from scratch.