@ryesenova The one you like and won't mind training after a day of dealing with other people's dogs.
But yeah - I'm trying to figure that out for NextDog. Right now, Laika (my shelter mutty mutt - 35lb tan mix of everything) is a huge partner in my training, particularly for reactive dogs. However, I never have her interact directly with my client's dogs. That, IMO, is just too much stress for a dog to have to deal with regularly, even the most stable one and I'm not going to put her through that. She's always at least 6ft away from the other dog, and if she shows stress, we stop.
But I would like my next dog to also be able to fill the role she does. My border terrier is a good "advanced" dog because he does have much stiffer body language and does sometimes stare if I'm not managing him.
As for what that dog is... ugh. I don't know. Overall as moderate of a dog as possible. My plan right now is to just keep my eyes open and maybe find one coming through the shelter.