http://www.fatglasspipes.com/Bones/t..._bone_pipe.jpg
just get something like that, usually cheap. nbd.
i look for the thickness of the glass, a thick pipe will be more durable.
size and shape of the bowl is another thing. if you dont smoke with alot of people you dont need a very big bowl. if the bowl isnt cupped enough and more v shaped its easier to suck ash through.
i always get pipes with a carb hole, its not needed really its just fun to puff the cherry and clear the pipe with it.
the rest is just looks, if you like it, its yours!
bongs/waterpipes are a different story, so clarify what you want