The Giedi Prime mission. Was quite hard, but Air Defense platforms, Sardaukar Elites, Advanced Carryalls and Inkvine Catapults saved the day.
As for the mission itself - it is pretty much Harkonnen Civil War. Baron Rakkam Harkonnen died, his senior son Copec took the throne, and you have the choice: siding with Copec (who resides on Giedi Prime) and the baron's younger son Gunseng (who oversees Harkonnen operations on Arrakis and opposes Copec's inheritance).
They all are evil so I sided with Gunseng; because a) he's not an arsehole like Copec; b) he didn't actually poison his own father (what exactly Copec did).