“Just Spiffy”
Or alternatively: “It’s just a spiffy day.”
Nobody will be able to figure out if you’re being serious or sarcastic.
My father, no matter what day of the week it is, says “Oh, pretty good for a Tuesday.” Obviously, if it is Tuesday, he’ll name another day of the week.
My father has a very dry sense of humour.
My six year old niece, bless her heart, always answers this question with a big smile and a hearty sincere “Fantastic!” that makes me want to grab her and hold onto the sunshine as long as possible.
Ississa Whoosh? A carpet? Not snug as a bug in a rug?
“Medium Rare.”
My sister in law calls and askes how I’m doing. I say fine, or good. And she will ask “what’s wrong”?
Huh? Apparently, if I don’t say excellent, or great, or top of the world or some other such thing, something must be wrong. Drives me nuts.
Hey, I use that! We got those two after y’inz were done with them, that line was the best part of their schtick.
The fact that no one’s ever recognized my response as being directly lifted from them makes their being cancelled for low ratings make a little more sense.
How about a mildly inappropriate Joey Tribiani response of “How you doin’?”
Well, you can’t make a horse drink.
A: How are you?
B: How am I? Well last night I was like a crazed rhinoceros of love.
A: Well that’s great!
B: No, she was great, but I was incredible, three times.
A: Really?
B: Yeah, but now I’m exhausted.
A: How are you?
B: Fine has powdered sugar.
B: Slicker than snot.
B: Cold! Colder than my first wife (husband).
B: Hotter than a two-dollar pistol.
B: Sick! Sicker than Michael Jackson’s wet dreams.
A:How are you?
B: Like your most secret passionate, dreams, but I am (married/straight).
Worse.
“5 by 5” is from ham radio? I had no idea! It’s something that Faith (the Other White Slayer) used to say. I haven’t spent all that much time wondering about it, but I’ve never seen an explanation of it … I guess I just figured it was something that the writer made up. The other characters on the show even talked about it once: someone asked “what does that even mean?” and no one knew.
So, what does it mean? 
My biology professor in college used to say, “Surviving” in answer to that question. One class, he pointed out that organisms with successful genes not only survive to maturity but reproduce as well. He pointed out that, as a grandfather, he had successful genes.
So when people asked him how he was and he said, “Surviving,” he meant “Surviving and reproducing.”
When performing a radio check, one responded with a one-to-five rating of the sender’s strength (ranging from “weak” to “loud”) and a one-to-five rating of the sender’s clarity (ranging from “garbled” to “clear”). Five-by-five, i.e. a loud and clear signal, was as good as it got. It became a general military (not specifically Ham radio) term for smooth sailing, smooth operation. If Joss Whedon first heard it in a movie, he likely got it from Aliens, in which the pilot says “We’re in the pipe, five by five.”
Dad was a ham is no longer with us.
One number indicates signal strength, the other the clarity of the signal.
Why five is the best I have no idea.
I usually respond: ‘Depends on who you ask’.
“Busier than a one-legged man in an ass-kicking contest.”
Excellent, thanks! That response has come to mind a number of times, but I’ve never said it because I didn’t know what it meant. Now I can’t wait until the next time someone asks me how I am (probably tomorrow at work).
You kinda did already. 
When I was very negative back then, I would say,
“Dying.”
Recently, it has improved to
“Not bad, but not very good either”
Delicious.
I usually say something like “Can’t complain! Well, I could, but what’s the point?”