After 25 years in veterinary medicine, I'm going to massively generalize here
I have see extremes on all sides of the spectrum and an ill-bred dog or one with a bad upbringing can happen in any breed.
My personal experience has been (keep in mind a vet office rarely brings out the best in a dog):
Chessie - I've always been cautious with chessies. I seem to have run into alot that have had a screw loose. If a lab and a chessie were in the waiting room and I had to bet money on which one was going to get spooky on me, I'd bet on the chessie.
Chocolate lab - one step below the chessie in spookiness. The most likely lab that would be a problem back in the surgical suite for catheters, nail trims, blood draws.
Black lab - generally easy to handle and willing to be handled. I've only met one truly vicious lab - one that would try to bite you if your back was turned. I've met a few fear biters but that is just poor socialization for the most part.
Yellow labs - big babies. also tend to be the more high-energy of the bunch. We had one, Jeter, that we eventually amputated his front leg because his fracture would not heal - he blew off 2 plates and broke numerous screws and pins. It did not slow him down.
Just personal experience and I've met dogs of all these breeds that I have fallen in love with and would have had as my dog in a heart beat.
Upbringing is so important with any dog. Good luck with your search!

I have see extremes on all sides of the spectrum and an ill-bred dog or one with a bad upbringing can happen in any breed.
My personal experience has been (keep in mind a vet office rarely brings out the best in a dog):
Chessie - I've always been cautious with chessies. I seem to have run into alot that have had a screw loose. If a lab and a chessie were in the waiting room and I had to bet money on which one was going to get spooky on me, I'd bet on the chessie.
Chocolate lab - one step below the chessie in spookiness. The most likely lab that would be a problem back in the surgical suite for catheters, nail trims, blood draws.
Black lab - generally easy to handle and willing to be handled. I've only met one truly vicious lab - one that would try to bite you if your back was turned. I've met a few fear biters but that is just poor socialization for the most part.
Yellow labs - big babies. also tend to be the more high-energy of the bunch. We had one, Jeter, that we eventually amputated his front leg because his fracture would not heal - he blew off 2 plates and broke numerous screws and pins. It did not slow him down.
Just personal experience and I've met dogs of all these breeds that I have fallen in love with and would have had as my dog in a heart beat.
Upbringing is so important with any dog. Good luck with your search!