My Guide Website?
There are many companies running tours in Berlin. Most offer regular public tours where you walk around with another 20 strangers ticking off the main sights off your card while you strain to hear your guide over the inconsiderate nattering of the other participants.
A much better, albeit slightly more expensive option, is to organise a private tour with Berlin Locals. They will tailer a tour to suit your specific interests and needs. Whether it is Prussian, Nazi, Communist, or post reunification history you are interested in, Berlin Locals will design a tour specifically for you. You also have a lot more flexabilty to explore with a private guide and it will move along at exactly the pace you prefer.
Prices are around €60 an hour, which may sound expensive, but if there are four of you, a 2 hour tour will only cost you €30 each, which is pretty darn cheap for such a personalised experience.