Here are the ones I have for my 50D along with my brief thoughts on them. I have ...
... the kit EF IS USM 28-135mm - I consider that my cheap walk-around lens. Gets good results without me being overly worried about banging it up a bit
... the EF-S IS USM 17-55mm @ 2.8 for indoors pics. Great for family events and such. It really is rather good in low light for people shots and it has enough zoom to frame without moving around alot. It is my most expensive lens (@ $1100) but has been well worth it so far as it is the lens that spend the most time on the camera. A downside though is that the lens will cast a shadow with the popup flash so it can be annoying the few times I forget to frame for cropping it out.
... the EF 70-300 IS USM. This is my least used lens, but for those few ocasions when wildlife or pets are in the grassland behind our house its come in handy. Sure it will be useful for nature shots during upcoming trips.
My rationale for picking these lenses was to cover the ranges I have taken pictures at in the past, and to get Image Stabilization because I dont have steady hands. Worth noting is that there is no real macro lens in there. Might pick up a cheap one at some point, but macro just is not something I do alot of, so who knows...