roll the hard six
Which one looks Like most fun as an agent?
plot deck? or the agent per se?
Rylan Olliven plot deck is real fun if you wanna mess with the heroes. you can make shop item 50gp more expensive, let them take one more travel step or buy one extra exp for the overlord once.
Mirklace makes Hot trait monster a little bit more meaner and has some fun faustian pack cards with the heroes which is a nice idea.
olliven deck is strong in buffing your monsters. some cards that could wipe out the heroes at the right time(one monster groups get "ravage" for a turn).
verminous deck lets do some interessting stuff monster with 4 health or less.