If you want a 6" screen, then the most feature rich PDF handling is available in: Onyx Boox 60, PocketBook 302, and Sony Touch Edition. They all include a zoom mode, and all have a touch screen (the PocketBook 301+ may be almost as good without a touch screen, and the Onyx has the clearest touch screen but it requires a Wacom pen). The Sony's zoom is not "sticky" (it resets after each page turn).
If you want a larger screen, I suggest waiting to see how good the iRex DR800SG becomes after its next firmware upgrade and also how good the upcoming 9" devices with a SiPix screen turn out to be.
For more information, see the
E-book Reader Matrix, which includes a row on PDF Capabilities.