Dish Network (DishHD) currently offers the best overall HD experience (most HD and best quality HD) of any of the providers; they are followed by FiOS TV who have the best quality HD, great service pricing...however, they are a new service and have limited service areas at present; cable generally does a pretty good job with quality, but they generally have limited availability...and the quantity and pricing varies widely based on geographic area and service provider (some provide a good HD package, others pretty dismal - some offer good pricing/hardware, while others are terrible); last is DirecTV...they provide substandard HD quality and quantity...however, they offer an ever growing number of HD locals and appear to ready to finally deliver on their promise to be the HD Leader - of course, they have saying this the past 18 months or more. If you're looking for a HD DVR...Dish Network providers the best (942/622), followed by DirecTV (TIVO), the FiOS and Motorola (6214?) appear to be on-par, follow by any cable provider using the SA8000HD/SA8300HD DVR.
The quality of digital cable and satellite channels (SD) vary widely...I recommend sticking with the provider who providers you with the HD channels you are wanting, gives you excellent picture quality, offers programming packages that meet your viewing needs, and offers you advanced equipment choices if that's what you're looking to rent or purchase.
This is my opinion based on my experiences. Currently, I am with Dish Network...but I will gladly subscribe to the provider who provides me with the best/most HD experience, reliable HD equipment, and best overall entertainment value.