The best way to know what you need to buy, is to know the limitations of your current kit; what is it about your current kit that you don't like? Don't just say "image quality", be specific: the 50D is capable of making stunning images and whilst the 18-200 doesn't have the best reputation, it isn't a total dog at all focal length-aperture combinations.
I am not writing this to make the old clichéd suggestion like "Ansel Adams could make a masterpiece with a box-brownie", but rather to get you to think about where the weaknesses of your current kit lie. If you are annoyed that your images at focal length xx are soft, then you know that this is an area that is important to you and that you should spend money to cover. If you complain that the 50D produces noisy images, this suggests that you are working at the limits of your camera ISO range and lens aperture: the 5D Mk.III will obviously help, but you'll benefit just as much from faster lenses; likewise, if you are struggling to get shallow depth of field. If you want 22MP to help you crop images, perhaps you need longer lenses.
I'm sorry if you were looking for a "just buy the xx-xx and xx-xxx" type of reply. If that's the case, then I'd suggest that you already know what you want and that you should just go ahead and buy it! All the lenses suggested on this thread vary from very good to excellent and I doubt any will truly disappoint. Just be aware that the more lenses you own, the more you'll end up carrying; I wouldn't want your photography to move from being fun to becoming a chore!