Except this isn't a double or triple battle. These are all 1 v 1 battles. This depends entirely on the skill of the trainer. All of Ash's regional teams have type and form (aerial and ground-based) diversity, and unlike Paul, Ash relies on counterstrategy/improvisation rather than setting up weather, entry hazards, or other effects for rest of his team to exploit.I view it as his strongest team in the sense that they work well together and each member battlea in a certain style that it covers their shared weaknesses. Could a team of Pikachu, Charizard, Snorlax, Sceptile, Infernape and Greninja beat them? Yes.
But in relation to that hypothetical team, there is no chemistry between those teammates except that they have all traveled alongside Pikachu. Like in real life sports, chemistry is important for teams to play well together and that is why they're his "ultimate (regional) team" since they all strive to push each other further along and to pick each other up. These six act like a family and they all continue on even if they don't have a shot at all.
It's hard to justify "ultimate [regional] team" when Pikachu and (Sato)Smogonfrog dominate the show. It's sort of how people claim DP had the best regional team, even though by the end it was mainly Infernape doing most of the work and a bunch of others being steady, but not great.