The company that we went through (and did another tour with again later in the week for a bike tour centered more around the art side of Berlin) was Fat Tire Bike Tours... I would recommend! The guides were great.
Neptunbrunnen - Neptune Fountain
The Berlin Wall:
(also known as the Holocaust Memorial):
We ended the tour in Pariser Platz - a square in the center of the city with the Bradenburg Gate and a number of countries' embassies: