Not going to make it a poll as I would expect that the possibilities are too great to accurately represent all of them.
I would expect there will be a lot of traditional ones such as: honey, sweetie, and darling. So what do you call your SO and if your brave enough, what do they call you most frequently?
Okay I’ll go first. I call my wife “babe” more frequently than anything else.
Darling.
For both of us, “Pooky” is probably the most common. I like it when he calls me “pocketfish”. He can’t remember why he started calling me that, but it makes me feel fuzzy 
She’s “little one.” I’m “dragon.”
With my first wife, I was “bunny.” She was “baby toad” (I picked it to show that the literal meaning of the words were less important than the intent. It was not a comment on anything – just two words I chose off the top of my head. She loved the name, BTW, and I asked if she would be bothered when I first used it).
I call him “Pup” a lot. He calls me “Pottamus” sometimes and then I kick him.
He calls me sweetie, cutie, and rabbit (we met online-dating, and rabbit was part of my username)
I call him cupcake and sweet-pea.
We call each other Pup.
I call him My Bones, because he helps give me form and structure - without him I’d feel like a puddle of protoplasm and goo.
He calls me Pie. Sometimes he will rhapsodize about what sort of filling I have, and whether I’m single crust or double crust or lattice crust.
My new boyfriend calls me “baby” which made me uncomfortable at first, but it’s growing on me. I mostly call him “babe”. One of my exes used to call me Sammi (my name is Sam) and it would just make me melt. I used to call him “boy wonder”…not sure how that one started, but it stuck for a while. At some point we dropped the cutesy names, though.
I’m not very creative - usually it’s “sweetie,” “honey,” etc. I try not to let on that I call everyone by nicknames like that because I’m getting senile and can’t recall their real names…
However, he calls me “Homie” sometimes, due to a funny interlude/misunderstanding. When we were on our honeymoon, Mr. Matata was snuggled up to me, with his face kind of buried in my neck. He swears that he said “I love you, honey,” but I heard “I love you, homie.” (For the record, we are two of the whitest middle-aged people you’ll ever meet.) Ever since, I’ve been Mr. M’s “homie.”
I call her “Pet,” “Petal,” “Sweetie,” and “Poops” or “Poopsie.” She calls me “Dear,” “Darling,” “Sweetheart,” and “Ok, knock it off.” 
Mostly, DH calls me *sweetie *or Mrs. Farthead and I call him *Roo *(his name is Roger) or Monkeyhead. No clue where these came from at all. My ex and I used to call each other Weez (again, no idea) or Weasel.
With friends I really like (and who can handle my sarcastic self), I am prone to calling them “Dolt” (always with love). I tell them if anyone besides me calls you that, it’s an insult.
Well, I have a bunch of slang sayings that I call him, but I call lots of men those, so I guess those aren’t pet names (Daddy, God, Killa, Nigga and Playa).
But I don’t have an actual pet name exclusive to him.
He likes to call me Fat Girl, but he says it, “Fot Gal”, which I love.
I call him honey, sweetie, or Vinnie.
He calls me bella, sweetie, or Sharon (that’s not my name.)
Hm…one ex used to call me “Little Sweet”, I mostly called her Kittencat or Kitty.
Oddest nickname I’ve ever had for a girlfriend was Waterpig. She actually liked it.
I probably split 50/50 between “babe” and “mama”.
I don’t really use pet names. When girls I’ve been with have referred to me by a pet name, I usually go over the top and start ending every sentence with a cutsie name as punishment.
My last girl called me ‘Daddy,’ as did the girl before that. But she was Hispanic, so it came out “Papi.”
Daddy/Papi/Sir, etc are the only pet names I’ll tolerate. 
“Babe”. I’m surprised more didn’t say this.
I call my boy “sweetness”. I haven’t told him yet that I got it from Stephen Colbert, as that’s what calls his gun (the one he has a romantic relationship with.) (what?)
I’m glad to know some of you call your SO’s “pup.” I have a dog and I call her “puppy” and for some reason I have a nearly irresistible urge to call my boy “puppy” as well. I don’t, tho.
I call him honey; I have no idea how it came about, but one day we realized how old a nickname it is (we’re in our early 20’s) and how stereotypical.
He calls me panda. Like the fuzzy animal. Mostly because I’m slow moving at the end of the night, after the gym and work.
I call him “Facehead” or sometimes just “Face” for short.