There are so many to chose from, the reality is that you can grow anything in a pot as long as you are willing to do for it all the things it cannot do for itself when in the ground. Provided you are haooy to do that, then the world is your oyster - as it were! Do you want evergreen? If you don't mind the smell, privet makes a wonderful pot plant, the bees and butterflies love the flowers, when grown as a hedge it gets cut and doesn't often flower. Get a nice tree book from the library and look at that, I would not recommend growing a tree that wants to be 100 foot tall in a pot (though I do have an oak tree that is 12 foot high in a pot, but I'd not do it again), look for trees that are naturally small, don't need alot of work and will give you the shape you want. Things like willow will grow fine but you will be for ever pruning it, the new smaller buddleas might be nice and smell sweet - mallows are good - the list is nearly endless.