It's hard to say, probably peep show. But I've already watched every single episode about 6 times, so it's already almost at the stage of getting boring. Perhaps Only fools and horses as there are so many episodes and even after 15 years I'm still not bored of it yet. I will probably go for OFAH, even though it's probably not my all time favourite, although very close.
I'd choose Prison Break. This is my all time favorite TV show because it's just the best. I love the whole thing. Every single episode was exciting and the story is very interesting. The main characters, the supporting characters we're all great and really did a great job in the show. I've only watched it once but I'm planning on watching the whole series soon.
Maybe Big Bang Theory because it's just slapstick comedy and tickles my funny bone all the time. I also like the characters very much. They jell well together and it's awesome seeing them in various comic situations. The dialogues incredibly witty and there's always something new to look forward to. As much as I like GoT, it's not a series I feel like watching for the rest of my life. It has to have a definite ending.
"The Prisoner" TV series, starring Patrick McGoohan! If you have never seen this 1960s TV show, you're missing out. It's about a ex-Government Agent [McGoohan], who quits but is captured and forced to live on a controlled village, where he if forced to figure out a way to escape each day and tempted to answer his reason for quitting his Government job, which he... well, you have to watch the series to know this answer. The other shows would be Simpsons, Monty Pythons, X-Files, and Bob Ross painting show [not joking about this show; used to watch it all the time when I was a kid].
I'd choose Smallville. I must admit that it's not the best TV series ever made but Michael Rosenbaum and John Glover were great. They are my all-time favorite TV show villains and I'd watch any of the episodes they are in without ever getting bored. It's villains who make stories great so if they are great, then quite obviously the show will also be good enough to watch even if the rest of the characters and story in general aren't all that great.
Probably The Sopranos. To me, it's the best show of all time by far. Just a great cast, great writing and a lot of action and drama to keep your interest throughout the whole series. There is also a lot of interpersonal drama and character conflicts that are really interesting and draw you into the characters psyche. If not The Sopranos, I'd watch Seinfeld. To me, that shows never gets old and I always laugh along as if it is the first time I've ever seen the show.
That's tough, but probably Red Dwarf. It is quite short, even though it ran for some 10 years, thanks to how British television works (short seasons and a lot of waiting in between them a lot of the time), but it's still one of my most beloved series of all time. Number two would probably be Monty Python's Flying Circus. I believe these are the only two series I've not only seen multiple times, but at least 3 times over.
Tough, but if I had to only choose one, it'd probably be Psych. No other show has every really given me so much joy from episode to episode. It's also one of the few shows where I couldn't wait to watch it each and every week. Such a tough question though because this is a lot like asking "If you could only listen to one song, what would it be?" There are so many different genres that's it's really hard just to narrow it down to just one.
