Summer, and it’s not close. UK summers are rarely unpleasantly hot, especially if you avoid the middle of the day. Whereas winters are damp, gloomy, and muddy. It’s rare to have snow on the ground more than a couple of days each winter. Plus, having to put on all the outdoor gear is a pain. It’s much nicer in summer when you can simply put on a pair of sandals and step outside.
Summer. Too many people don’t shovel their walks, and I prefer not to have to worry about slipping on ice.
Winter.
South Carolina summers are hot, humid, and buggy. Winters are lovely - cool, rarely really cold, almost never any snow, low humidity, and a good mix of sun and cloud. Absolutely the best outdoor weather.
I prefer winter because, when you run, you make your own body heat.
^^^This is totally me as well.
I love winter, but in colder temps I’d rather be on skis. I’m not great in super hot humid weather but I still walk long distances regularly in the hottest months.
Very, very early winter.
I mean, we call it autumn, but “early winter” will do for the OP.
summer. can just go out. No need to bundle up first. & it’s always dark in winter after work.
I choose both in defiance of the OP. 
OTOH, where I live, “winter” means the temps are usually in the 70’s / 20s.
I recently came up with a decent metaphor for temps in the low 60s or lower. It’s like standing barefoot on sharp gravel.
Ever second of the time you’re in that environment every bit of your body exposed to it is complaining loudly. It’s completely unrelenting; there’s no getting used to it. It might not be excruciating or injurious, but it’s simply miserable every second and you can think of almost nothing else but wanting to get your feet off the sharp gravel ASAP.
And the colder it gets from about 65, the more painful and miserable.
I said summer up thread, but if I lived somewhere humid (which I never would), that would totally change my opinion.
Alright, who’s holding him down & who’s hitting him? Please form two orderly lines!
a south FL coldsnap is like sweatshirt weather. 
Winter (Montana). I’ve got a good beard and am fine down to -20F. Kahtoola micro-spikes have been a game-changer.
Probably summer – as I can usually wear what I am wearing in the house (other than putting on shoes). If I go early enough it might not be too hot (though there are days it is still stinking hot at 4 AM)
On the other hand:
Winter especially after a snow can have a nice quiet stillness, and the cold air can be refreshing as long as it us not SUPER cold. Plus I get better views of the Mississippi with the leaves gone
I’ll still say summer, but not a huge gap.
Brian
It can get down to -40 in the winter here, so definitely summer. Although fall and spring are better.
I prefer summer (in Canada) definitely. My wife prefers to restrict our summer walks to mornings and evenings, which is fine with me.
Where we live, summer is pleasant. Mid 70s and no humidity. Winter is dark and it will be raining. So, I pick summer.
Yes. I prefer walking in winter or summer.
Summer is nice, don’t have to dress up.
Winter is also nice. Air is crisp, scenery can be stunning, and there’s a feeling of accomplishment after coming in from a 12km walk at -31C that you just can’t get in summer.
Photos taken New Year’s Day, 2022, on aforementioned 12km walk at -31 with heavy hoarfrost turning the city into a sparkly wonderland.
NE MN
I’ll take summer any day. Even if it’s hot and humid. The only days I miss walking the dogs is if it’s too cold (usually -10 or colder). I walk them in the morning so it’s very rarely too hot in the morning for them to walk.
Winter walking involves long underwear, snow pants, heavy socks, boots, jacket, reflector vest & head lamp (because it’s dark), hat, sometimes a face mask, neck warmer, mittens or choppers (depends on the temp), also depending on the sidewalk conditions I may have to wear ice cleats on my boots.
Summer involves none of the above. Maybe a light jacket.
It’s a conundrum for me because summer is so much easier - just put on some shoes and out I go. But I HATE being hot. I hate the sun in my eyes and sunglasses or a hat don’t suit me for various reasons. Bugs suck too.
I’m way more physically suited for winter but I gotta do the boots and the coat and the gloves and usually a ski mask, and if it’s dark (which it usually is) I wear a hi-viz vest, and if it has snowed I put a protective wax on the dog’s paws. And picking up poop in the dark & in the cold is a nightmare.
I walk almost every day of the year unless it’s under 30F or over 80F. I guess the best time for me to walk is in the summer when it’s 65-70 and preferably shady.
I prefer winter, because it’s just so much easier to manage my body temperature when the air is cold. Also, I love snow, so snowshoeing is a favorite.
But I walk all year, I can’t afford not to. In the hot months, I sometimes get up at 4 to go walk, because the air is the coolest then. One of my favorite walks includes a steep grassy hill, and I like walking up and down it, over and over, as a way to keep my hiking legs in even when I can’t stand hiking because anything over about 65 is too hot to hike in.