By a mon army, I assume you're going for a single mark/God army. Effectively, it gets dicey at that point. First, which mark do you want to go with? This determines what sort of Troops you get, as Daemons don't have any choice if you go with singlemark.
Bloodthirsters kill a lot, very effectively in HTH. They are low toughness, low save.
Daemonettes hit a lot, very fast. They are low str, so damage less, low AP/Rending, and low toughness and save.
Pink Horrors hit slow, not very much, are low T and low save. But they are all psykers, and grant your enemy many Feel No Pain saves after burning them with psychic fire.
Plaguebearers are slow, low attacks, tough, Durable, low save, extra cover, and can have FNP. Probably the best of the troop units, but then you're talking a Nurgle army.
AS for a mono-list, I see two types that MIGHT be competitive. Mono-Khorne and Mono-Nurgle
Mono-Khorne
Bloodthirster
Herald of Khorne x4, 1 on Jugger.
BloodCrushers x5
Bloodletters x10
Bloodletters x10
Bloodletters x10
Bloodletters x10
Bloodletters x10
Bloodletters x10
Fleshhounds x10
Fleshhounds x10
Flying Full Deamon Prince of Khorne
Flying full Deamon Prince of Khorne
Soul Grinder.
Mono-Nurgle
Great Unclean one
Herald of Nurgle x4, one on palanquin
Beasts of Nurgle x5
Plaugebearers x10
Plaugebearers x10
Plaugebearers x10
Plaugebearers x10
Plaugebearers x10
Nurglings x10
Plague Drones x4
Plague Drones x4
Full flying Daermon Prince of Nurgle
Full Flying Daemon prince of Nurgle
Soul Grinder of Nurgle.