It all depends on what you intend to do with your photography. Shooting a sports event will require something different than shooting macro shots of flowers in your backyard. And believe me, I'm not an expert on every lens out there. I can only speak for what I've used. And Nikon lovers--I'm afraid I haven't used Nikon at all, so I really can't offer any opinions there.
However, if I had to choose just one lens to recommend that Canon photographers buy, it would be the 50mm 1.4 lens. This is for several reasons:
1. It's moderately priced--right now about $400 on Adorama.
2. It's incredibly fast--1.4 aperture means great for low light.
3. It produces clean, sharp images.
4. And it's lightweight--an easy walk-around lens to keep handy.
Now, Canon also offers two other 50mm lens models. There's the 50mm 1.8 II that sells for about $120, and then there's the 50mm 1.2L, which sells for about $1,450.
I also own the 1.8, and there is a vast difference between it and the 1.4. The 1.8 is completely plastic (except for the glass), and it's not nearly as sharp as the 1.4 when stopped down to 1.8 aperture. If you're going to get a 50mm lens, I would really recommend just going ahead and buying the 1.4. (I bought the 1.8 first and later bought the 1.4). As for the 1.2, I've heard nothing but good things about it, but I haven't actually used it. I'm sure you can find plenty of reviews about it on the web.
And, since a post isn't any good without a photo, here are a couple of my willing model, Astin, taken with the 50mm 1.4 lens.
See the nice bokeh it produces?
Astin: "I'm sure I saw a cat out there..."