Well, okay I just did the math and for me chain shirt with large shield is the best route i would say. Not only will it offer me the highest bonus, but also it will allow me to get another feat which you can never have too much of. (most likely exotic weapons for bastard sword or katana:D)
The 19 dex even for rangers and rogues is quite a harsh prereq. (-> would say elves and half elves and some sub races)
The other option the single weapon wielding seems nice however I have two questions regarding this. If my char has a candle, torch, book or something that is NOT considered as a weapon, will the bonus still apply or not? (Or practice baton for instance!)
Also...one thing if I know correctly currently if your character is using a single 1 handed weapon you still get TWD bonuses to your parry. Will now the TWD bonus and the Duelist bonus be mutually exclusive if for instance a char has both, and only the greater one will apply?
Oh also for Two handed weapon users, so if someone runs around in a chain shirt and uses a Two hander they still receive with improved parry 5 ac from parry right?
Either way Shield parry CS+Large shield still tops everything. 4 ac from parry +2 shield and 4 from magic vestment (or 3 since those varnishes are NOT that hard to aquire!)
Shield Parry Agrippa's Fundamental Guards Greater Two Weapon Defense
number of feats required 1 3 3
prerequisites: Dex 15 Dex 16+, Int 14+, BAB +11 (and feats) Dex 19 (and feats)
possible max AC: 10(with mv) 9 (only with Leather Armour!) 9 (only with Leather Armour)
additional bonus: shield property? offhand non weapon item bonus? offhand weapon property, and damage (and/or active parry skill!)
For me still the most usefull seems to be shield parry.
Greater TWD gives you more attacks, however you will still have that -2 AB for each attack. Also would need to have those fitting weapons for those hands, and not to mention helps a lot if you are an elf, a half elf or a subrace with dex bonus at start. <- any chars not of these races that are rogues, rangers and possibly bards, should just either go Agrippa, or Shield Parry. In which regard Shield Parry is already more usefull and less demanding.
On the server there are very few off hand items that make it worth to use Agrippa not to mention the 3 feats you need to waste, and the two attributes that are above average requirement, or could also say dex could be considered quite high as a prereq. (Not to mention if you already have 16 dex why not shield parry?
)
So my final judgement is still shield parry tops all.
And Agrippa is the weakest option.
Agrippa currently has the same prerequisite to top your AC as TWD. Basicly the Leather Armour. Technically it offers the same AC bonus, however only if you use Leather Armour, which means that you are better off using TWD then.
So i would suggest to give Aggripa's route a bit of extra slack. Like for instance if you take this route the armour you are wearing has lower ACP considered for the 30 parry.
Here is what i mean in an example:
Bonetti's Defense beside the +1 AC it also negates 0 point of ACP from the armour you are wearing that is counted from the 30 parry.
Thibault's Geometry: beside the +2 ac it also negates 1 point of ACP from the armour you are wearing so now you can use studded leather armour to get the max parry AC bonus.
Agrippa's Fundamental Guards: beside the +3 ac it also negates 2 points of ACP from the armour you are wearing, allowing you to use it in chainshirts to gain the max parry AC bonus.
Since for Agrippa the prereq is 16 dex would make sense since Chain Shirt offers +4 ac from dex bonus.