I'll take a stab at this, with a caveat. I have found dog people to be some of the most opinionated people I've ever met, and answering this question could raise some hackles (pun intended), particularly among those who have favorite breeds.
Having said that, the short answer is that my German Shepherds and my Austrailian Shepherd/Border Collie mix have been the easiest to train for me, followed by a Whippet that did an impeccable recall and heel. Trailing behind would be my Dachshunds and a Chihuahua that I fostered.
I am a firm believer that any dog can be trained using positive reinforcement, but that different dogs are motivated by different things. Some dogs are more "treat" motivated, while some are more "play" motivated. Hence, for some of my dogs I've used treats as a lure and reinforcement, and for others I've used a tennis ball.
Even while factoring in trainability, my mini Dachshund, Rusty, was my "heart dog"; the one that left the deepest, most lasting impression. I had Rusty for 15 1/2 years and I still miss him terribly.
On a lighter note, it used to crack me up when Rusty did a down/stay. His short little legs made it tricky to determine if he had indeed, followed through. The visible difference between a "stand" and "down" was barely perceptible to me, particularly since I'm nearsighted.
Some Dachshunds can have an independent streak, but they are trainable. The most important thing that I have found with Dachshunds is that mutual respect goes a long way toward building a successful dog/handler team.