I would have to agree with most of the replies and say that Migros Klubschule is the best value for money on average... if you are looking to get started right away or if you are looking for an intensive class.
I would also recommend
looking into the adult education schools and programs offered within your community (if you live outside of the city center). I live in Dietikon and the classes I've taken at the
Berufbildungszentrum are not only the least expensive I've seen anywhere, but also the best quality I've had. The drawback is that the classes are not offered in an intensive pace, the class size is slighter larger (but over time it decreases to a smaller size) and they only start 2 times per year. Besides the price, the advantage is that they teachers are very seasoned. My teacher has been teaching German for 30+ years and has developed her own amazing tricks and tips to help us learn.
One thing that I find all German classes are lacking is training in pronunication! I find
pronounciation training especially important in Zurich because all I hear is Swiss German and I am trying to learn high German. I also want to minimize my American accent. I am tired of trying to speak German to strangers and to be answered back immediately in English!
The best class I have taken is offered at
EB Zurich- Aussprache Training Deutsch. The class is only 80 chf for 8 1-1/2hour weekly lessons. There is nothing cheaper or better! I would NOT recommend the class for beginners, but only to people that are at the B1 level or higher. You need to be comfortable reading German out loud and holding simple conversations.
The class is absolutely fun and so much better than sitting through grammar training again! The teacher is very passionate, kind and engaging. Plus, as an added bonus, she is also an English teacher, so she understands the struggles of English speakers learning German.
My final recommendation is to
take multiple types of courses at a few different schools than just putting all of your eggs in one basket by just taking a standard German course. You will benefit from different teaching styles, subjects and methods.