I recommend lemon grass. It's grass so it's easy to grow in a container and possibly best to do so as it won't take over the garden that way. If you don't have a green thumb this one will take absolute neglect and zero effort and you can grow other plants from it. I decided to grow it for cooking as I like the flavour (common ingredient in Thai food).

I snipped off the ends and made tea from it when I had the flu. I'm not that into herbals and I was verrryy sceptical, but to my surprise, it was a great relief for flu and fever symptoms. Kind of like taking aspirin. For more on what it can be used for
http://www.buzzle.com/editorials/9-2-2006-107399.asp
http://www.answers.com/topic/cymbopogon
http://www.morethanalive.com/Lemongrass-cut

I may not take other herbs for their medicinal value but this one is a keeper.