Google - make sure that you download the Google AdWords editor, it will make your life much easier
Google is by far the biggest in the market. This can mean that you may get a better deal with others as they seek to prise business away from Google. However, whilst Google retains its pre-eminence in search, it will provide you with more traffic. On the other hand, you may be able to achieve top spot on other networks for much less and, by virtue of your top position, you could receive more traffic.
There are other PPC networks to consider too:
MIVA (formerly FindWhat)
The platinum rule in PPC is that you must: test, test, test.
Google Analytics will be of great assistance in this area.