Which lens (two extremes)?

Just getting started with my XTi, and thus far have the Tamron kit lenses (28-80, 75-300), the nifty 50, and a Sigma 18-200 (for traveling light). Ultimately, I know I'd like to get Sigma's 17-70, their 10-20 and Canon's 70-200 F4L. Unfortunately, I can't do them all at once My question is which one would you get next? I love to shoot all across the board (landscape, portrait, etc.). Thanks for your advice!

My next three lenses also. I'd go for the 10-20 since you've got good coverage on the rest already. UWA is all kinds of fun.

The 70-200 f/4 is the best lens in your list but you already have that range covered - AND you are new to the camera - so I voted for the 10-20. UWA will fulfil needs you didn't even know you had. the other lenses just replace stuff you have already got, they don't add anything new

I put in another vote for the 10-20, you wont regret it since you like to shoot landscapes. And like xpsentity said, the UWA can be tons of fun and add a unique touch to photos

I want to vote for 70-200 but as you said landscape then i will never think of telephoto zoom long and only think of WA lenses, so sigma 10-20 is the best choice.