We read this question at least once a week in any photography forum. While there are a lot of possible answers, the first question should be “What do I want to shoot?”. It is only once you know what is the kind of subject that interests you that you can pick up the perfect lens.
So to help you clarify your thoughts on the subject I made a list of the most useful lens, grouped by categories of subject/style. Note: most of these lenses are on the expensive side since they are specialized lenses mostly targeted at pro.
If the only thing you want to do is walk around and take pictures, flexibility is key so you definitely need a zoom lens. While some people would argue that the EF 24-70mm f/2.8L is the perfect walking lens, I find it a bit too short and I rather recommend the excellent Canon 24-105L f/4 IS. Indoor or outside, this lens has the reach and width to get great shots.
Generally speaking, portraits look best when the background is blurry. While the 50 mm f/1.8 might do a nice job, its short focal range means that you have to be close to the subject which could distract them. This is why lenses such as the 85mm f/1.8 (or 85mm f1.2L) or 135mm f/2L are preferred. While a lot of people suggest a zoom (24-70 or 70-200), I personally think it is better to have a fixed focal length in a controlled environment since since the image quality is better, the lenses cheaper and you can always zoom with your feet.
While the full frame sensor of the 5D might not be suited for wildlife photography because of its shorter reach compared to cropped sensors, its video mode might interest people who want to film the behaviors of animals. Depending of the kind of animal you are expecting to encounter, you can pick either the 300Lf4IS or the 400mm f/5.6L. While the 300mm is good for anything bigger than a dog or not easily scared, the 400mm is the minimum required reach for small birds. Both of these lenses have incredible image quality and focusing speed.
When you shoot a building, it is rare that you have a lot of room in front of you to position yourself, generally the farther you can go is on the other side of the street. That is why wide angles are popular among architecture photographers. While the first lens that come to mind for such a task is the 17-40mm f/4L or the more expensive 16-35mm f/2.8L, if you are really into architecture, you have to take a look at the tilt-shift lens, such as the TS-E 24mm f/3.5L or the TS-E 45mm f/2.8. Tilt-shift lenses allow you to do more than just correct the perspective, by shifting the lens you can create a mosaic of pictures that you can later on combine for a perfect panorama. This is why even at 45mm, you can take pictures of buildings right in front of you.
When people think about landscape, the first thing that comes to their mind is wide-angle. Well, they are right. Lenses such as the 16-35mm f/2.8L and 17-40mm f/4L are classical landscape lenses. But limiting yourself to a wide angle would be a terrible mistake! Every serious landscape photographer carries in his bad a… 100-400mm f4.5-5.6L IS lens too! Surprised? You should not be. having a long focal length is the only way to create a perspective compressing effect that looks so cool when shooting mountains, or to take a sunset pictures with the sun taking half of the frame.
The line up of macro lenses is not that impressive, only 3 models are available from Canon for the 5D. The 100mm f/2.8 Macro having the most flexibility. While the 180mm f3.5L Macro is incredible to shoot bugs, it has limited uses outside of the macro world. An interesting alternative to macro lenses, if you are shooting still life, is to use a TS-E 90mm f/2.8tilt-shift lens. While the macro effect might not there, this is the preferred lens of food and gem photographers since you can tilt the plane of focus to have the whole picture in focus.
A cheaper alternative is to use extension tubes. While I have never been able to take a good macro shot with these without using a tripod, they offer the best bang for the buck and are easy to carry around.
It is hard for me to advice in this category without knowing which sport you want to shoot. The general line of thought is that you need wide aperture and a zoom unless you have complete control over your position relative to the subject. In this regard, the 70-200mm f/2.8L is a great chose for most activities. Dont mind the IS. In sport the action is going on so fast that IS would never help you to stop it down.
Wedding photography equipment is pretty standardize: 24-70mm f/2.8L and 70-200mm f/2.8L IS. These are the two lenses you will ever need. But if you are interested in weeding photography, you probably already know that.
I hope this article helped you to organize your thoughts about the subject. If I missed an obvious pick, please leave your suggestion in the comments.