“I am that guy.” Amos, The Expanse
Context: He just talked down a mild-mannered scientist from killing a bad guy, telling him “He’s not that guy.”
Does that line support or oppose my contention that context is key?
Buddy Hackman: “You want to know the secret of comedy? Go on, ask me the secret of comedy.”
Johnny Carson: “Okay, what’s the secret of…”
Buddy Hackman: “TIMING!”
Buddy Hacket, not Buddy Hackman (I think). Oops.
Oh and…
Ok, one last time. These are small, but the ones out there are far away.
Father Ted (explaining the concept of perspective to Father Dougal)
Father Ted
Series 2 Episode 1
I like that one.
“You do what you have to do. You take the consequences of your actions”. Barney Miller.
“Book 'em, Danno” - Hawaii 5-0
“Just one more thing…” - Columbo
I don’t think either needs much context. But these are used multiple times in each show, they are not lines used once.
I’d argue those belong in the list of best catchphrases not best lines
Because I don’t want to spoil it for anyone who hasn’t seen the classic Honeymooners episode in which Jackie Gleason’s character is a contestant on a TV quiz show, I’ll give the penultimate line leading to his response:
Host: Who is the composer of Swanee River?
Ralph Kramden: …
Hackett.
Never mind.
That prompted me to search for it.
https://www.youtube.com/watch?v=VpwM7HipPVk
Is there context here, because I don’t get why that’s funny?
Not a list, but two that I always wait for when I run across the episodes scrolling through channels:
Buffy Season 2, Episode 5 (Reptile Boy), after Angel tells her there romance isn’t a fairy tale, no matter how much she wants it to be:
“When you kiss me, I wanna die”
Law & Order : Criminal Intent Season 3, Episode 4 (But Not Forgotten), where Isabel (Alicia Coppola) gives a final farewell to her corrupt husband, who killed her hit-man husband in order to romance an marry her:
“I wouldn’t raise a dog with you.”
IMO, a line that is truly great doesn’t need context. It is either great on it’s face, or it creates context in the beholder’s mind (even if the context it creates is not the one from the TV program). IOW, if you gotta explain it, it’s not great.
Garak’s monologue to Sisko in the same episode:
“That’s why you came to me, isn’t it, Captain? Because you knew I could do those things that you weren’t capable of doing. Well, it worked. And you’ll get what you want, a war between the Romulans and the Dominion. And if your conscience is bothering you, you should soothe it with the knowledge that you may have just saved the entire Alpha Quadrant and all it cost was the life of one Romulan senator, one criminal, and the self-respect of one Starfleet officer. I don’t know about you, but I’d call that a bargain.”
Two from Captain Jean Luc Picard form Star Trek: The Next Generation:
“It is possible to commit no mistakes and still lose. That is not a weakness. That is life.”
“When one has been angry for a very long time, one gets used to it. And it becomes comfortable, like…like old leather. And finally it becomes so familiar that one can’t ever remember feeling any other way”
Neither of your posts are about single lines, though.
I’ll see you this, and raise you with a line from Fresno, the infamous '80s spoof of prime-time soaps like Dallas:
Foundling Teri Garr, confronting a lineup of men in clown suits: “All right, which one of you bozos is my father?” 
If you want context, you really have to watch the entire episode. Here it is in a nutshell, along with the full quote:
Trent is (almost) the only one left alive on an Earth that has been invaded by the alien Kyben. Everyone else has vanished, and nobody knows where or how, including Trent, who has big gaps in his memory. He is guided by a glass hand that has been grafted onto his arm, which talks to him – its a computer, but it can’t tell him the answer, because it’s missing critical data storage, which is kept in the three fingers missing from the hand. As Trent recaptures the memory lobes friom the Kyben, one by one, he learns more (the lobes/fingers plug in like USB sticks). The humans have stored their memories and body information and can be reconstituted later. They set off a series of “dirty” bombs to make the Earth dangerously radioactive for a thousand years. After the Kyben die off or leave, they can be reconstituted. Trent and a woman he comes across fight off the Kyben and succeed in killing them off and preventing others from coming, and in doing so gain the last memory lobe. Trent asks the Hand where the peopl of Earth are. They’re in a wire stored inside his body. Trent is actually a robot, and was unaware of this. The woman recoils in horror, as Trent realizes that he will be alone fo a long time.
- Control Voice: [closing narration] Like the eternal man of Babylonian legend, like Gilgamesh, one thousand plus two hundred years stretches before Trent. Without love, without friendship, alone, neither man nor machine, waiting, waiting for the day he will be called to free the humans who gave him mobility, movement, but not life.
I think it is more a spoof from Laces II, where Lili asks three men, “Which one of you bastards is my father?”.
Yes, I watched both.