What is the best race for a druid?

There is no "best" race for a druid, but Nightelves and Tauren do give different advantages.

Nightelves start off with 18 strength, 25 agility, 19 stamina, 22 intelligence, and 22 spirit.
Tauren start off with 26 strength, 15 agility, 22 stamina, 17 intelligence, and 24 spirit.

The most important stats for a druid tend to be pretty much everything. Each of the two races has a healthy mix of these so this is personal preference. Tauren get warstomp (stuns nearby enemies for 2 seconds, 2 minute cooldown), endurance (5% more hp max), cultivation (+15 max herbalism), and nature resistance (+10 nature resist) for their racial abilities. Nightelves get shadowmeld (stealth while out of combat and immobile), quickness (dodge chance + 1%), wisp spirit (25% faster ghost speed), and nature resistance (+10 nature resist). Just remember that when picking a druid, pick the race that appeals most to you, and remember that Tauren are the Horde and Nightelves are the Alliance.