Meraxis, god of bliss, is incredibly difficult to gain favour with compared to most of the other gods. Has no favourgaining pickups (ie, nothing like the small spider pickups in the poison realm). Been to this realm twice, and still have less than 350 favour.
Tartarith, god of punishment, is also harder than most of the others.
Regalis, godess of poison, is perhaps a bit on the easy side, because of all the available pickups.
Just my take on this so far.