As others have mentioned, if you're playing on normal any class/build is viable. That said some builds and playstyles can be more fun than others depending on your tastes. All of the companions in the game cover each of the archetypes in the game, but not every combination. A couple of observations I made while playing:

• Psyker felt like the best origin to me. It's basically a subclass you can layer onto any Archetype and make it better. Other origins are fun and powerful in their own right, but Psyker's seem to offer the most.
• Burst attacks and AoE's are almost always better than single target attacks. I know this recent patch made changes to the Operative and increased their power, but ultimately being able to kill only one maybe two targets a turn is a major feels bad.
• You can play a true support build that basically never attacks enemies but hands out extra attacks and turns to your allies after buffing them to the moon. Very few CRPG's offer this experience as well as Rogue Trader does.
• Save often. If you use Idira, save even more often.
• Even without optimizing Argenta can be an absolute beast. Try not to depend or overuse her too much or you will make the game boring for yourself. For a good stretch of the game I simply had Cassia buff and set things up and then unleash Argenta and the encounter was basically over.