The Punisher: The Slavers (Garth Ennis, 2006)
The Punisher takes on a group of Eastern Europeans running a prostitution/slavery ring in New York. This is definitely as dark as the series/character ever gets, some of the slavery experiences described are very upsetting and The Punisher's kills are more chilling than ever too...but you have to be on his side of course.
The Punisher: Barracuda (Garth Ennis, 2006)
After the incredibly depressing slavery story line, Ennis goes back to the fun ultraviolence of the character by creating his best villian. Barracuda is a strong and violent yet crazy and hilarious hitman who is let loose on Frank Castle. The fight in part 3 is brilliant.