short answer is all of them. The 501 508 721's have no PVR fee most of the rest under normal circumstances have PVR fees payable to dish. I personally am using one of each.
921 and 942 have a DVR fee
ViP622 has DVR fee and also "HD Enabling fee" if you don't subscribe to DishHD package.
But these would give you lots more hours of SD recording time than the SD models. Maybe not what you want because of the cost but there you go.
The 721 is probably your best choice since it's a dual tuner with no fee.
Even an old DishPlayer (7100 or 7200) would work fine. Those can be upgraded pretty easily. I actually started with a couple of those, upgraded to 120GB disks.
DISH has ceased using the "PVR" moniker. All new and refurbished receivers with that functionality are referred to as DVRs as are the fees associated with them.
I have a 508 that I bought on e bay. It gets the job done. One day I hope to upgrade to a dual tuner so that I don't have to choose between which shows to record when there is a schedule conflict.
Dvr is definitely the way to go. I can't imagine tv without it.
HOOORAY, I got my 508 pvr, hooked up and running! Man, I love it, now I choose when I watch my programs, good to be in charge again! What could be better, a 508 pvr, and no monthly FEES!