I don't think there's a 'best' system, just a best fit for your needs. It may be worth trying all of them in your environment to see if you can do all of the things that you need to do.
I've been using Sendmail for about 6 years. It's configured by using a macro command file that is processed by a macro interpreter (m4) to generate its config file. However it's not the easiest to learn how to configure - the O'Reilly Sendmail reference (http://www.amazon.com/gp/product/156...lance&n=283155
) is over 1200 pages. Also, there's more than one file containing info about how it operates (although they're usually all in /etc/mail).
Recent releases are secure and it's used reliably pretty much everywhere. I recommend it because of that and also because it is working well here with Cyrus' saslauthd.