The leading proxies are Burp and Zap. Burp pro edition just about has the edge on Zap, but Zap is completely free. The free edition of Burp is usable, but lacks advanced features like the active scanner.
Paros and Webscarab were once leading proxies, but they are no longer cutting edge. Zap is actually a fork of Paros, but it has moved on a lot.
Squid is a proxy, that's true, but it's not an interactive proxy. It's not much help for testing a web app.