This is always an ambiguous question. If it were just "what are top 5 favorite TV programs that are not concluded" (in other words, not including Seinfeld, etc.) then that is pretty clear cut for almost everyone.
However, once you add "HD", then there are several permutations:
- Shows that you like, but look about the same in SD or HD * (e.g. "24" and "Terminator: Sarah Conner Chronicles")
- Shows that are mildly entertaining, but look spectacular in HD (e.g. "Lost" and "Prison Break")
- Shows that would be still be excellent in SD, but make really good use of HD to be better yet (e.g. "Heroes" and "Rome")
- Shows that would have zero viewership if they were not in HD (e.g. "Get Out" and lots of stuff on Discovery HD Theater like "Sunrise Earth")
- Shows with often poor writing that become a little more palatable in HD (e.g. "Torchwood").
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* Yes, everything looks better in 16x9 720p than in 4x3 480i - but many shows like "24" that are usually set in dark and grey warehouses or HQ/offices that might as well be dark warehouses - don't benefit as much as brighter, more colorful shows set in places like Hawaii...