The TXL bus is always crammed with passengers. I do not recommend taking it for this reason. And it also takes a long time to travel into the center of town. May I suggest taking the X9 bus from Tegel to the Zoologischer Garten train station. It takes about 15 minutes to get there, and it only makes four or five designated stops along the way. Once you are at the Zoo Bhf, go to the regional train platforms. Twice an hour, a regional train goes from there to SXF airport station. The ABC zone ticket you purchase at the airport is valid for only 2 hours, and this would be the quickest journey IMO. I do not recommend using the S-Bahn mainly because it takes a long time. And you will have to transfer trains at the Ostkreuz S-Bahn station (which right now is a major construction project). The S9 train does not run on the main city line any more (since the Ostkreuz station reconstruction began). It makes up part of the eastern section of the Ringbahn.