Hi,
If you want to be completely anonymous, the proxy MUST check as a "high anonymous elite" proxy (aka Level 1).
What is the different between the different 'levels' of proxies, you ask?
A transparent proxy will show your real IP and the IP of the proxy.
An anonymous proxy uses a special HTTP field called 'VIA' that reveals that you are using a proxy (but doesn't show your IP).
A high anonymous proxy completely hides your IP, only showing that of the proxy, and does not give off a 'VIA' flag.
I have heard that Proxy Multiply is good for finding proxies

(bias alert)