Blizzard just introduced as Hall of Fame feature to it's Armory webpage, it will keep a permanent record of the top team after every season:

The Arena Hall of Fame recognizes the best teams of each Arena season – those teams who finished in the top 0.5% of their team bracket in each battlegroup. These teams and their members have been permanently enshrined in a special Hall of Fame section of the Armory.In addition, every team member who played in at least 20% of his or her team's total games also receives an armored Netherdrake mount and a special title as trophies.