Not actually sad, just with a sad face. Obviously Bassett Hounds are number one, with Bloodhounds close behind. We had a lab who was good at looking sad, and a beagle/heeler mix who couldn’t look sad if her life depended on it. Now we have a lab/Shepherd mix who is looking sad because I won’t take him out when he just went for a walk.
You nailed it in one. When I read the thread title, bassets immediately came to mind, and they’re undoubtedly the saddest looking. Bloodhounds have similar eyes, but they also look threatening, other than bassets with their smaller build and short legs.
Spaniels don’t look sad as much as worried. They’re definitely concerned.
It’s one of their endearing traits.
My late and lamented dog, Orson, was a lab but he was The Saddest Dog in the World.
We used to joke that he had been in the cover of the “Sad Dogs” magazine a record number of times.
I mean look at this:
I don’t know if it’s sad-looking, but dachshunds are masters at putting on the poor, poor pitiful me expressions. “I know I ate just an hour ago, but I’m starving.”
Beagles have those big moist eyes, always look like they’ve been cryin’ 
It’s just camouflage, they’re usually also thinking “I’m going to kill you if you cross me!”
Ha! You know your dachshunds (teckels)!
Not mine. “I’m cute! I’m lottsa fun! Play tug of war and I’ll win!! Walk? I’m on it! Food? Yippee!”