I don't think I can even count the number of dogs I've had in my lifetime. 3 of them really stand out for me though.
The first was an Irish Setter that my parents got as a puppy when I was born. We grew up together and she stuck with me like I was her puppy, I can remember waking up early in the morning when I was 4 -5 years old and going outside to go back to sleep with her on the patio or in her dog house. My parents would wake to find me there, she and I were inseparable.
The second was a Brittany that I got from a kill shelter intending to foster him until the rescue could find him a permanent home. We foster failed almost immediately with him, I don't know what it was about him but, there was just something special that anyone who spent any time with him would see almost immediately. The poor guy had been neglected and abandoned by his owner and was never socialized or allowed inside, it took a few months to get him somewhat comfortable with us and a year and a half before he really felt at ease, I can still remember the first time he came behind the kitchen island to see my wife, it was a huge step for him. Once he felt comfortable though, he was one of the gentlest and most loving dogs anyone would ever want. Unfortunately, he had a degenerative neurological issue in his spine and developed dementia so we lost him way too soon. Medications gave us an extra year and a half of quality time with him, and then one day he gave me that look and I knew he was ready. Letting him go was one of the hardest things I've ever done and I still tear up thinking about it. We gave him the best possible life we could though and he died on his property under a clear blue sky never having the stress of being with a vet at the end. As hard as that was for me, it was the last gift I could give him and a debt I felt I owed him.
More recently, we had a Border Collie that just showed up in the fence line one day while I was out working. She was only with us for about 9 months before passing after surgery to remove and intussusception in her intestines. She was experiencing renal failure when she got to the vet so we knew the surgery was a crap shoot but, if we were going to lose her, I wanted to lose her trying, it was a huge blow to lose her after surgery though, I thought that if we lost her it would be in the OR. She showed up just a couple of months after I was in an auto accident that I suffered a brain injury in and while I was going through the scariest part of my recovery, not knowing if I would every fully recover my speech or if I would forever be left with a stutter that made communication difficult, to put it mildly. She was a very observant and empathetic soul that seemed to always be there to comfort me when I needed it most. The most striking example was one afternoon when I was talking about my fear over what the future held with my wife and Tricksie was asleep in the room. At one point, I broke down in tears and she woke up and immediately ran to me and hopped up to wrap her front legs around my waist and hug me. I realize that I may be anthropomorphismizing her actions but, this sort of thing happened often with her and her contact and presence when I most needed support was exactly what I needed then, I would have had a much harder time making it through that time without her. Losing her was a huge blow that I still feel daily, her absence is huge hole.