Beauty is in the eye of the beholder...so I have no clue what would be a good looking wheel to you.
Are you wanting suggestions other than the 18" wheels that BMW offered for the X series? Are you considering aftermarket 18" wheels?
I run the Style 153 18" wheels in the winter mounted with dedicated winter tires...and I like the way they look. Here's a link to the BMW X wheels...remember that the e70 & F15 wheels have a larger hub (74.1mm)...and the e71 X6 has a split axle...the front is 74.1mm but the rear has a 72.56mm:
All BMW wheel styles X | BmwStyleWheels.com