For security? The most secure email service I can think of is run through www.riseup.net - You have to apply for an account because it is an anarchist server, but if anyone can penetrate that, they deserve a hacker trophy from the president.
The best free e-mail for security is Zoho Mail. This is because it has a solid email service with ample storage. However, there are hackers who can get to any email service. You should be careful on what you send or save.
Define security. In my opinion, hushmail is the top of the food chain. If you go with hushmail, be sure to use it so you must use java, otherwise your passphrase is decoded on server side. The scope of security is likely to go well beyond this question in order to actually tell you how to be secure data wise. I recommend looking into lastpass for password security. I also suggest looking into PGP for instant messaging. Look for a program called pidgin and an addon for them that uses PGP. This will allow you to use encrypted IMs. Always use a clean machine, meaning no virus, preferably not on windows. And possibly most importantly, use a SSL VPN, this is 10 times more important if you are on an unencrypted wifi network. Without all these other details, your perception of security is a 100% waste of time.