for the druid class i cant really say just one mod...there are quite a bit that are useful for many classes
The big mods for druids that i use are:
-Outfitter(or i guess itemrack or any of the other wardrobe mods...i like this one the most)
-Bartender 3(removes the blizzard action bar graphics and bars and gives you bars that you can modify and add keybinding to easily)
-TrinketMenu(lets ya switch around trinkets and easily assign keybindings to trinkets)
-X-perl unitframes(changes how you see players hp so its easier to read)
-Ctmod /w Raidassist (Basically all classes really need and i reccommend going to
www.ctmod.net for updates for it)
-MCP(master control panel, allows you to enable or disable addons in game as well as set up character profiles
-EquipCompare (lets ya see what item you are wearing when
-Mobhealth3 (lets ya see approx. hp of enemy)
-Druidbar(shows mana in shapeshift forms but i cant find an updated version)
just to name a few