Seems to be the general view that the Zenith has the best performing receiver but many people have audio dropout problems which Zenith can't (won't??) fix. After that there is the Hughes E-86 (also sold as the Mits and the Toshiba DST-3000 - they are the same box, built by Hughes). Then there is the DTC100.
Sony has an STB which has been discontinued.
There are new HD-capable STBs with Direct TV and OTA due out this fall (probably means by early 2003!!), from Samsung and from Hughes. They may be worth waiting for.
I have a Hughes E-86 which I am very impressed with. I think you will probably be OK whichever unit you choose.