I can't think of any better oils for Winter than these 3.  Clove, Cinnamon Bark & Eucalyptus.

Clove is quite possibly the best oil when it comes to killing off nasty germs that lurk in the winter.  

Cinnamon Bark is absolutely divine!  It's such a warming smell, perfect for diffusing with the fire going to create a calming environment.  Oh, and it also packs a punch in the germ fighting arena.

And finally Eucalypts.  Winter just isn't the same without the clean fresh scent of Eucalyptus.  Clear the airways and help fight colds and flus with this Australian native.