I used to use the really large AirBake sheets (by WearEver I think). I'd get 3-4 rows of cookies on them. The cookies in the middle rows, however, always needed an extra 2 minutes or so to bake compared to the outer rows. Then, they started to warp on me. When I posted this on Baking911, Sarah Phillips suggested switching to the non-insulated, non-shiny, heavy pans. (Williams Sonoma 2/$30) These have worked beatifully for me, and I don't think they'll ever warp. The only drawback is that because they have sides, you cannot slide the cookies off of them.
I'm just sharing my experience ... I know others can recommend sheets that work really great for them, too (so no offense taken Zamode). That's the great thing about this forum ... you can so much great advice! Happy Baking!