This is a tough one, since all four races "make sense".
Night Elves (the canon druid ofc) and Tauren are the hippes of the druidic ying and best fit (in my mind) as balance/restoration through their reverence for nature and suchlike, whereas Trolls and Worgen are the darker, more aggressive yang as befits the feral druid. Both races are, from a lore perspective, shapechangers .. Worgen from the curse and Trolls via their tribalistic voodoo worship.
If I was to pick the "best" race from a lore perspective, I'd go with:
Night Elf - Balance.
Tauren - Restoration.
Worgen - Feral.
From an in-game perspective, I'd say Tauren. Our (not a typo forms look great and we have comic appeal when hopping around in cow form. Plus, we get to pick flowers fast and have a bit more stamina than other races.