Cavs, Brussells, Cockers, Poodles or Yorkies are "high maintenance" in terms of grooming (Cavs not so much, although some altered ones get coats that approach altered Cocker-y) but IMO not so much in terms of being good pet dogs. I see lots of Cavs, Cockers, Poodles and Yorkies in average pet homes who's owners do just fine with them despite doing very little training and low to moderate exercise.The other breeds can be fairly high maintenance in terms of being pet dogs, depending on what the owner's expectations of having a pet dog is.
As far as breeds I have had....
Considering several people who have Belgians told me PyrSheps were "too hyper to do anything with", I'd say PyrSheps are pretty high maintenance by most people's standards. Savvy is certainly not as easy as the other herding dogs I have had. He's high energy, a bit hyper and a bit destructive if left to his own devices. And he jumps fences...which in itself makes a high maintenance dog as far as I'm concerned. Also they require a lot of ongoing socialization, training and interaction....which I don't mind because that's what I enjoy doing with my dogs anyway
The Belgians...it depends on what you find high maintenance or not. IME they're good house dogs for the most part, although some can be rather needy. They don't tend to be super destructive and can be happy chilling out with you, providing they are getting the exercise/interaction they need. But they do require a lot of early socialization, early training and a certain amount of ongoing interaction. They aren't a dog most people would be happy "casually owning". The owners of the Cavs, Cockers, Poodles and Yorkies I see...they'd probably not enjoy a Belgian so much. For an active owner who wants to have a very interactive relationship with their dog (training, exercising, etc) I don't find them to be a really a hard breed. Or maybe I've just had them so long they seem perfectly normal
At the same time though, I know they can be prone to behaviors that are dealbreakers for many people. Guard-y, suspicious and/or reactive behavior towards strangers/strange dogs. Shrieking with excitement or frustration. Obsessive behaviors. And just plain quirkyness LOL GSDs can be high maintenance for similar reasons to Belgians and some of their own. Two of my GSDs were rather high maintenance - one because he was a "no touch" sort of dog when it came to strangers and the other because she had to be separated from the other girls in the house. Of course, there are super low drive, low energy, oddball GSDs (and Belgians) out there who are extremely easy too. So some of it depends on the individual dog too.
Ziggy is crazy but a really super good house dog. I'm sure people who see him in agility probably could never believe that LOL
Coat care wise, I've not had any really "high maintenance" breeds. My Collie's coat got higher maintenance once he was neutered but he didn't have a lot more hair than my boy Belgian. The Belgians, if their coat texture is correct are pretty low maintenance for hairy dogs. Savvy is almost no maintenance except he gets mats behind his ears and under his front legs. He gets burrs too, although they brush out really easily.