My choice : Sendmail most used, well known, stable ,very portable, and powerful..
And even if my opinion is biased (Im a sendmail administrator)
I think sendmail isn't so complicated to configure/manage through the m4 macros.

Alternative choice : Postfix
A little bit simpler and said to be more secure and faster
(even if I'm not aware of any Senmail vulnerability in the current version...)

In fact it depends on your objective :
If you want something fast and easy to install to do simple things I think you should use postfix.
Now if you want to hook some stuffs in your MTA (lots of already developped add-on exists),
Or if you know you'll do complex things with your MTA, try Sendmail.

"Trying to be a SMART lamer" (thanx to Merlyn ;-)