I use qmail as my MTA (with the ssp patch) + Spamassassin, ClamAV, CourierIMAP, and Horde.
I have also used Postfix in the past, and like that as well. I have two major complaints with sendmail. First is no support for Maildir's. Maildir's are much better then mbox, especially if you happen to use NFS. Second is the amount of people that seam to run open relays because of bad sendmail configs. If you use sendmail, make sure your not an open relay.
What originally drew me towards qmail was security. Qmail 1.0.3 came out in 1998 and still hasn't been hacked.