european trip. what ONE lens to bring with a 40D

Go with the 24-105. I took my 50, my 17-40, and my 70-200 earlier in the year to europe... 95+% of my shots were taken with the 17-40. The 24-105 is a very versatile lens.

Definitely 24-105 if you go for one lens, if you go 2-lens combo then bring your 10-20 too

24-105L

For a crop it doesn't make any sense to choose any of the lenses designed for FF IMHO, if you are restricted to one lens. Get a 17-55mm 2.8 IS. It gives you much wider angle than the standard FF walk around lenses will produce on a crop, plus some reach to 55mm. Plus 2.8 which I use very frequently. Plus IS.

I'm surprised no one else has mentioned it yet.

Your UWA and a fast wide/medium prime is all you need.

d44 wrote in post #8340545
Unless you are really burning to buy a new lens, and using this trip as the justification, I think you are set with what you have (sorry!).

i was gonna sell my to sigmas to fund an L purchase but i guess it would be good to just take all 3 lens and get a new one when i come back?

I'd definitely say the one lens best for Europe would be a 17-55 IS. That focal range is great and you need some wide there; the 2.8 plus IS would be fantastic for museums/churches/night stuff without tripod.