Nikon Fieldscope 50mm ED. I'm impressed every time I look through mine, and I have two of them. I have both the 13-30x variable lens and the 16x WDS fixed lens. The fixed lens provides a larger visual image and has good eye relief for eyeglasses. The variable lens provides more power with a good image, it's just smaller visually.
Best I've ever used is the 55mm kowa. 15-45x, and all the X's are usable. Lots of other small scopes max at around 30x. Noticably better than every other sub-65 scope I've ever used, and equal to or better than many larger scopes.