Never been to Berlin, want to be where it is at. What airport do most travel into?
Both are good and have good links to the centre of town. Back in the day One airport serviced east berlin (Berlin Schönefeld Airport) and the other the west (Berlin Tegel Airport) when The Wall was still up. Since the old devide and borders are gone and much blurred, and many places like kreutzberg (east) are being seen as hip it really makes no difference where you land In relation to the areas of interest. Get the best deal and enjoy berlin!
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I was just there last week :) I didn't fly directly there (road tripped from Westphalia), but I know that Berlin Tegel Airport is the main international airport and I would've flown into there had I been coming specifically from San Francisco.
If you're looking for where to stay, Mitte (literally meaning "middle") is the most happening area.
It is quite easy to catch public transport from either airport- go with whichever has the best deals. Find out more about how to get around Berlin and what to do here: http://www.neverendinghoneymoon.net/berlin/
Berlin Tegel Airport is a more "normal" airport, while most of the flights departing from Schonefeld are on low-cost airlines. In Tegel there are lounges available and fast-track security. Unfortunately Tegel is not connected to the S-Bahn-System so you have to take the bus (TXL express) or grab a taxi.