By the way: Your connection via Jungfernheide with two changes involved takes 1:12 at the best, the other one with two changes via Beusselstraße 1:15 at the best. Via Charlottenburg it takes 1:18 with only one change.

The theoretically quickest and maybe most convenient and most reliable (but boring) connection is

• five minutes on the 109 or X9 bus from Tegel Terminal to Jacob-Kaiser-Platz subway station (one stop on the X9, I believe three stops on the 109),
• change there in comfort to the Subway (U-Bahn) in the direction of Rudow subway station (from there you can already see the aiport),
• change at Rudow to the X7 feeder-bus which will take you in six minutes directly to the Terminal without any need to walk from the trainstation to the terminal

The only disadvantage: 45 minutes of 1:03 hours is on the subway cross town whereas the S-Bahn or the regional trains give you a (really) scienic ride through the West and the historical East of Berlin with several great sights (BTW: Another disadvantage of the connections via Jungfernheide or Beusselstraße: You won't see anything interesting)