"Best" is indeed subjective. I owned a Tivo HD DVR when I was with D*, and now have the 622 with E*. I have also had some experience playing around with my friends' new D* HD DVR as well as the one used by TWC. I know nothing about the rest. My unofficial opinion is:
Tivo - Pros - has the best graphics, most intuitive interface, and some of the best features (like "wish lists" and "season pass" capability). Their remote is also well designed. Cons - no TV2 output, extremely slow to change channels or page up and down (I heard this was a D* software limitation, not Tivo), and expensive if you have to pay an extra monthly fee for having a guide (which is crazy!)
622 - Pros - love the TV2 output with complete DVR functionality and the remote is very ergonomic (I prefer it to my Harmony which I paid $200 for). Cons - lacks some of the cool features on the Tivo, and the graphics are very elementary (compared to Tivo's, its like the difference between looking at 1970s PacMan vs today's Call of Duty). I would think this is purely software, and could be fixed easily if E* made it a priority. It's obviously not.
I know far less about the new D* HD DVR. It has better graphics that 622, but not as good as Tivo. I wasn't impressed with the remote...seemed cheap. Also, at the time I tried it out, it lacked many of the basic features we have even on a 622...but I have read that many of these have been added in recent software updates.
The TWC box - just a piece of crap. Horrible interface, clumsy design, just awful...I threw the remote at my brother-in-law and said "If I had TWC, I would have them out here today, ripping this piece of crap out of here!".