I laughed when I read a list of "healthiest dog breeds" that included Chihuahuas. Just today I saw 3 chihuahuas at work (vet hospital) that all needed expensive workups for complex medical problems, and all were under 10 years old. I won't say they're not good dogs, since a lot I work with are generally very nice and patient, minus the occasional hyper-aggressive tiny ball of fury. I just wish people would stop breeding them to have a tiny round head with barely any mouth space to fit all their teeth into.
The breeds I hate working with (not that I hate them, just that I dislike meeting at the vet hospital): pugs/other brachycelphalics, shih tzus, and large working breeds no one socializes. I don't intend to offend anyone who owns them... But pugs/other brachycephalics are genetic wrecks - if you find one in the shelter who needs a good home, there's no shame in that! But to breed pugs or buy a pug from a breeder is just continuing a line of animals who can barely breathe their entire lives for the sake of "cute factor." Shih tzus tend to be incredibly mean to strangers (not to their owners, though, so they can make great pets for the right person!). When we hear "shih tzu" at work, we instinctively reach for the muzzles. Large breeds, especially German shepherds, I've noticed, tend to be either amazing friendly pets (25%) or fearful and aggressive dogs only the owner can get near (75%). I've met a 120 pound German shepherd that, during an appointment with, the vet would leave the exam room door open behind her so that she could sprint out if needed, and wouldn't even be able to touch the dog. "Oh, what a cute puppy!" except they don't realize the importance of training and socializing a puppy that's going to get really big really fast and potentially aggressive!
Pit bulls are 99% love bugs. I have met one and ONLY one aggressive pit bull, and even she was sweet as could be as long as you weren't around food. Recently we saw a pit bull torn to shreds by a neighbor's dog that jumped the fence (neighbor's dog was completely unharmed, the pit bull was so submissive) and she patiently sat still while we rinsed her multiple 6-inch-wide and 12-inch-long flesh wounds before anesthetizing her to stitch her up. Today we saw a pit bull that spent a year and a half (!!) at a shelter waiting to find a home, and was the most cooperative patient we saw all day. They are all rock stars about letting us trim their nails!