I think you created yourself a paradox....
It depends what you mean by 'best'


Exchange is very popular which even I have to admit makes it best in popularity....

The real prob is if you want REAL email servers windows land has a few but they ARE REALLY EXPENSIVE. Making a secure server isn't trivial in WinDowsLand

In the *nix world there are sendmail, postfix and qmail....
However email in linux land is either much simpler or more complex, depending how you look on it....
That is its more modular so you can deliver with one method and send with another etc.
In this respect qmail is probably the 'simplest' since it actually does everything, by using IMP (www.horde.org) it becomes php compatible.