Usually Goldens and Labs, but we have worked with many breeds.
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.