I inherited a Maggoth lord from a generous friend who didn't want it anymore and I've been struggling to decide what to do with it. I agree with what others have said, Bloab seems to be generally considered the best choice and his spell can be used to hasten the removal of a troublesome enemy unit/monster with a lot of wounds. When partnered with a Beast of Nurgle who can do damage when retreating, it's possible to have his spell deal damage during Movement, Hero, Combat and Shooting phases. A cool combo, but beasts are another model that is considered very weak for it's point value. Horticulous Slimux can also damage when retreating, but can't then charge in the same turn...
I'm tempted to build it as Morbidex though. Nurglings can be difficult to remove and he can replenish their numbers each turn. Even with Morbidex's buff they won't be doing much damage, but that's what Morbidex is there for with his range that can reach the enemy from behind a shieldwall of Nurglings. Not very competitive maybe, but for friendly battles it sounds like a lot of fun!
What I like about both of these options is that it takes another model that isn't very impressive by itself and gives them a chance to shine! Orghotts seems pretty lacklustre in comparison. The buff I really want to make the Maggoth Lords more valuable is to simply give them the Demon keyword so that they can activate the Locus abilities for Plaguebearers and Plague drones. I'm not hot on the lore but the Maggoth Beasts sure look like demons to me...
