I currently have these lenses : all Canon, EF-S 15-85 IS , EF-S 10-22 , EF 50/1.4 , EF 85/1.8, EF 200/2.8 L .
I currently use a 550D ( T2i ), and occasionnally my former 450D ( XSi )
the lens I use most is the 15-85 IS.
If I had to choose only one lens, it would be the 15-85 IS. The IS image stabilisation works wonders, also very good in video mode. There is some vignetting with that lens but the 550D/T2i ( and later models too, I presume ) makes an in-camera correction and the vignetting disappears.
The 10-22 is excellent . But if I had to buy a superwide angle today, I might go for the Sigma 8-16, though I dislike one thing on it, it only starts at f/4.5 , and in stormchasing situations we tend to find ourselves in rather dark light.
Nick, keep in mind that the 50mm/1.8 will give you 80mm FF equivalence, that is almost the same as the teleposition i your 18-55 kit lens.
Here's my recommendation : get the 15-85 IS.
Plan B : look at the prime EF 35/2