Poodle. Poodle, poodle, poodle.
Yes, I am biased. ;-)
More seriously, here is a list of dogs considered hypo-allergenic.
List of Hypoallergenic Dogs
If you are serious about training your dog to be a hearing assistance dog, it might be worth it to pay a professional to help you evaluate a dog's or puppy's temperament and suitability for that before you buy it. Not every dog, regardless of breed, has the characteristics that you would want.
For instance, schnauzers can be incredibly yappy. Many of them will just bark bark bark at everything they see. That doesn't help you if want hearing assistance, because you want them to bark once or twice and then shut up! And come get you, obviously.
You want a dog who will bond very well with its owner; who responds to training; who likes to be praised; and, obviously, who can hear very well itself.
In poodles, a high retrieving drive is highly correlated with trainability. A dog who retrieves easily and naturally, who brings the toy back to you after it's thrown, is a dog who likes praise more than it likes playing with the toy itself.
For my two, as a comparison, Casey is a natural retriever, and will nearly always bring back to me anything that I throw. He is the larger of my two.
Pippin, sweet little Pippin with the face of an angel, will grab the toy, run off and shake it so he can "kill" it. He is not at all interested in bringing anything back to me, unless I have on hand a very high-value treat. He will do ANYTHING for bit of chicken or cheese.
Guess who does better in his obedience classes - it's Casey, by a long shot.
Both are miniature poodles, Casey from a good breeder (who has bred lots of champions), and Pippin from a rescue.
Both of them have passed their "Good Canine Citizen" test (from the AKC) and have the certificates to prove it. I would recommend that any pet owner at least train their dog to that level; the requirements are very, very basic, and any pet should be able to do them (sit, stay, down, come, accept touch from a friendly stranger, be left with a friendly stranger without panic or upset, see strange things like an umbrella opening, a cane, a wheelchair, without reacting in fright).
Have fun and take the time to evaluate and pick out the right dog for you. If you do it right, you will reap the rewards for years to come.