Description Economy: Floor Plans

I think the best format for dungeon adventures, if you can pull it off, is a map with all the key information written right there in the rooms on the floor plan. 

Put as much information as possible on the map. I tend to use pencil, so I can erease and change as needed:  

• layout of the rooms and corridors  
• light, show where torches are burning and lighting the room
• elevation if it is not all on the same level; flowing water, pools, stairs, ramps and shafts, ladders, elevators (and if they go up or down)  
• unusual ceiling heights, height lines
• doors indicating locked or barred, in which direction they open; secret doors, portculli and gates, illusory walls, tapestries and curtains
• traps: trapdoors and other traps (spear traps, crossbow bolts, falling rocks, gas), and teleporters
• fixtures: fireplaces, pillars, statues, altars, thrones, wells, cave-ins, magic circles, levers, stalagmites and stalagtites
• maybe even furniture: beds, shelves, desks, chests, chairs, benches, stools, barrels, cupboards, armchairs, couches, braziers, carpets

If there are several levels, if possible insert a cross-section or side view. This helps you understand how they sit in relation to each other, especially if there are multiple connections between levels. 

If you have space, put a terse random encounter table on the border or in an inset, and show monsters in the rooms. For the different groups of monstes, the "factions" that are fighting for dominance, have an idea how they stand in relation to each other. You do not need to write this down you are clear for yourself, unless you want to share/publish the adventure.

It is easier to transport more information on the map if you use coloring (for example for level of depth, light, to indicate type of ground like water, flagstones, sand, or to show materials like brown for wooden furniture). But you can get far with just black and white symbols. It also is easier if you do not use crosshatching or solid black to indicate rock. They may look nicer, but take away space for your notes and descriptions.

There are commonly used symbols for this and using consistent ones, this costs you very little space. For example, Dungeon of the Mad Mage (5e) has a full page of such symbols, many other adventures have symbols too. If you always use these symbols, then you do not need to have a key on the map for yourself, they will be self exlanatory. Stairs are a bit tricky, be sure to indicate where the end is that is on the current level, and if they go up or down, either with a small dot or an arrow pointing up the stair. Here are some common symbols I tend to use (secret doors can be combined with doors if they open in a direction):

Or take a look at the key from Dyson Logos who also offers fantastic free maps to use.

Here is an example dungeon from me, with the encounters right on the key, and a vertical view. Here is how David Bowman designs his floor plans.

As for the layout of the floor plan, there are good discussions about linear vs. open design. The open designs that were used in early megadungeons are non-linear, they feature multiple entries, loops, and alternative routes through the dungeon. The poster-child for this style is Caverns of Thracia by Paul (later Jennell) Jaquais. Nonlinear dungeons lead to more exciting play, as they give players many meaningful decisions and allow room for more tactics. 

However, not every adventure site is a megadungeon. Most of my old simple crypts and wizard towers are pretty linear, while my castles naturally are much more open in where you can go, because that is the nature of such edifices.

If there is too much information to fit on the map, for example for a complex tricks or traps or riddles, one of the next best formats for me is a one-page key, where you put encounters, and special rooms, traps and magic items on a single facing page. This way you still will have everything in view.

Gary Gygax recommended to not put the key on the map to make it easier to fix mistakes. It also helps to modify the map and provides an overveiw over the treasure amounts and monsters. He put it on a separate one page sheet, one liners of monster groups and their treasures. His early maps were so densely popluated with tiny rooms and corridors with literally no negative space, it would have been difficult to write anything readable on that map, which is maybe how he came to the separate page originally and naturally. But for less dense designs, you have quite a bit of space to add pencil commentary.