Without a doubt the Urban Decay, Oz the Great and Powerful Glinda Palette which I recently reviewed with swatches.
It has really made me experiment more with eyeshadow and I think I have come a long way with it as now I can do a smokey eye effect. I even bought the Real Techniques Starter Set for eyes so I can experiment further with good brushes, so I think I could experiment more with the looks achievable with this palette. I find it ideal for a bright day look or I can smoke it up for a sexy night-time look.
This palette is only a limited edition so I may have a tear when I hit pan! The other good thing about this palette is that I can refill with Urban Decay eyeshadows, so I will be going nuts picking out new shades. Plus I am still eyeing up one of the Naked Palettes which is on a wishlist of mine :)