A short answer might be, "We use rescues, so it could be anything".
A slightly longer answer is "We usually choose Goldens and Labs and mixes of Goldens or Labs for service dog work."
But both those answers have a LOT of exceptions, and in general, how an individual dog scores on temperament testing is much more important that what breed they are. One problem is that there isn't a specific breed that has been bred for generations to be service dogs. Instead, there are a few, rare individuals that could be good candidates for service dog work (we estimate that about 1 out 1000 dogs is a good candidate for this job). We wish there were more breeds (and more individual dogs) that made good service dogs, but other breeds don't have a good track record.
If you want an even longer answer, check out our essay on the breeds dogs we use.