I'd say the 3 easiest and most likely tiers you would see teams fighting to go undefeated would be rookie, sophomore and vet. With these 3 tiers you never have to play against a higher tiered team. So any team that went undefeated between Seasoned-Pro tier would be very impressive, since they would most likely need to beat a bunch of higher tier teams along the way.
I think the Stunners current undefeated record is much more impressive though. They have built their team and climbed all the way to Vet while dominating all the way to the #1 spot. In rookie, you can win a lot of games with gameplans alone. The Stunners have dominated with Builds, Roster Construction AND great gameplans.