You have many needs here and one lens isn't going to do it all.
Possibly a nice walk around lens is your kit lens or a Canon 17-85 IS.
Certainly the landscape is best served by a wide angle zoom. You could use your kit lense or the 17-85 or go for an even wider angle lens starting at 10-12mm.
Usually portrait is considered to be 80mm in film terms, or about a 50m lens with cropping on a rebel XT. Your kit lens has the 50mm focal lengthas does the 17-85 I mentioned.
If you wanted to do 50mm in a prime lens, youcould get theCanon 50mm F1.8 prime which is a steal at about $80 US. It's not a "pro" lens but it's a good lens.
Beyond that, if your looking at pro lens applications, you have to be really specific about the type of work you want to do, and get recommendations from pro's that do the work
For instance, if you really plan topro portraiture, then seek out pro's who spend a lotof time doing portraits (studio?) like a wedding photog or some of thefashion photographers on this site.
good Luck! Terry