I have been playing Shogun 2 for a long time (not into FoTS), and now I play it more like an RPG game, upgrading my generals and choosing the most powerful retainers for them. The No.1 retainer to me is of course Sword of Amaterasu, if my favourite general doesn't get it, I won't continue the campaign (that's why I play on VH, I need save/load). I noticed that there are some new retainers, compared with the version I played before, so I just wrote them down while I repeatedly save/load to get the Sword. And I also noticed that the retainer list created a long time ago was not complete even at the time it was created. Now I post a new one, and seperate the retainers into different categories, according to their effects (except some exception). I will keep updating the list depending on whether other players are interested.
1. General Ability Boost
Oyabun +15% increase to influence radius (4-star retainer, it works better with Stand and Fight, Rally, and Instill Fear)
Master Drummer +20% to the effectiveness of Rally (4-star retainer, helps you hold the line long enough)
Taiko Drum Master +10% to effectiveness of Rally (3-star, unless you already have Master Drummer, otherwise don't pick it as your first choice)
Sword of Amaterasu +10% to the effectiveness of Inspire; Instills fear in enemy units (5-star, my favourite, consider you need to reach level 6 and spend so many skill points to get the instills fear trait, and now you get it at level 2 in the early game when most units are ashigaru, you surely want to get your hands on it; besides, you can put some points in the skill Religious to boost Inspire, so your 40 bow ashigaru will suddenly become 40 bow warrior monk)
Legendary Meppu Instills fear in enemy units (4-star, you can get this one only in the last retainer slot)
There is some misleading info about Sword of Amaterasu, before I start another category, I want to clarify first. 1, both daimyos and generals can get this item; 2, it only appear in the first retainer slot, if your general didn't get it, you will never get it for this general, but you can always work on another general; 3, according to most players, the effect of instills fear doesn't stack; 4, I believe it's an unique item, if you see an AI general has the Sword, kill him.
2. Mistress (this is one of the exceptions that is not arranged according to effects but names)
Ambitous Mistress +1 to command when attacking on land; -1 to loyalty
Foxy Mistress +1 to command when defending on land; -1 morale for all units
Discreet Mistress -5% to the chance of being assassinated
Over-ambitious Mistress -1 to loyalty; +10% to campaign movement (4-star, I think most of the mistresses are rubbish, giving a slight advantage in auto-solve battles or avoiding assassination, come on, I usually do the battles myself and enemy ninjas usually aim at monks not generals. But this one is different, increasing campaign movement give you a huge advantage in strategy on the campaign map, that's why I give high ranking to those retainers boosting campaign movements. Loyalty is not a big issue in Shogun 2, there are lots of ways to increase generals' loyalty)
Joro +1 morale for all units; +5% to the upkeep
Gorgeous Mistress +1 to command when defending on land; -1 morale for all units (exactly same as Foxy Mistress)
Soldier's Mistress -5% to the chance of being assassinated (same as Discreet Mistress)
Common Mistress +1 to command when defending on land; -1 morale for all units (same as Foxy Mistress and Gorgeous Mistress. what's wrong with CA, creating so many retainers with identical effects!? I need some retainers boosting reload skills, running speed in battle, etc.)
I will just stop here, in case I will waste my efforts and no one will be interested.