Great or best, highest in quality and value.

Paper product used as a barrier against unwanted messes, such as the liner at the bottom of a bird cage.

Of high quality.

Cream of the crop, of extraordinary quality.

The phrase is derived, not from the position of premium bottles in liquor stores, but their position behind bars. (as opposed to "well" liquors, which are stored in the bartender's speedrack well, and "call" liquors, which are popular but not displayed (and which must be "called").

The most expensive liquor in the bar, located on the top shelf because it is rarley ordered.

When someone scores a goal in hockey and puts in the top of net, it goes "top shelf". See also: top cookie

The best of the best. Usually refers to where wine or liquor are kept in a specialty liquor store. The best stuff is kept at the top for display until someone needs it.