It depends on how wide you want to go and how much you're willing to spend.
The Canon EF 28mm f/1.8, which I own (I also use the 5D Mark II), is a great prime lens and not too expensive (~$500 new). It's small, light and fast. It's my lens of choice for shooting landscapes, sunsets, etc.
The EF 16-35mm f/2.8L is a well loved ultra
-wide zoom, but it is large and costs more (~$1700 new).
The EF 14mm f/2.8L II is even wider (I have the original version), but costs more still (~ $2200 with current Canon promotion). This lens is best for getting up close to something to distort the perspective. It has a convex lens, so you can't put a filter on the end of it, so watch the fingerprints and be careful not to walk into things with it!
For a deal, check out Canon's website, where you can buy refurbished ones (w/90 day warranty) or the buy&sell forum on fredmiranda.com, where pros and amateurs alike can buy/sell used photography hardware.