I'm not quite sure what you mean when you say untraceable. Are we talking about the proxy host being able to see everything you do, or someone being able to track back to you through a proxy?
As for the most secure proxy, there are many secure proxies out there. The most secure (in terms of data being safe) one would be one you own. In terms of anonymity, you don't want to connect to some proxy servers such as ones owned by Private Proxy, because they, like many others, monitor everything you do for "illegal activity" while giving the false sense of privacy.
Honestly, your best bet would be getting something like Tor, as long as you aren't going on any websites that call for you logging in to them while connect to the proxy. Otherwise it's pointless.
“True hacking is like skydiving, you want to make sure you have arms, because nobody’s going to be there to pull the chute for you.”