Imo, i could be wrong tho,the fastest way(grind, boring, Argh) is the amatz basin challenge mission,basicly what u need is 1 assassin ,2 assassin heroes
and to run the perma sf build,then bind your hero skills and yours to the numpad so for ex:when u press 1,4,7 it activates[deadly paradox],2,5,8 activates[glyph of swiftness],3,6,9 activates[shadow form].All u have to do is go to your position and start renewing shadow form till the time expires
http://img89.imageshack.us/img89/9011/gw232rv8.jpg
i gained around 10k faction after that run in 21 minutes ,take note this is a mindless and boring way of gaining faction.
Other form of gaining faction that imo is quite fast is vanquishing and can be done with no cons and in averege times of 1 hour,1:30h depending on your level on experience and skill,and if go with people or using hero/henchman.Problem with vanquishing is that it gets tedious after a while, its fun to do all echovald forest once ,twice on 1 char but it gets boring after that.If u get help form guildies and employ pve only skills,cons and such u can achieve really fast times for example i had a 30 minutes clear on eternal groove with a physical heavy team.
Other way to be considered are the story books but i found them inferior to vanquishing or hffing in amatz ,reason being :u need all 13 misions in hm in Cantha for 120k faction points reward towards title(note that its points not faction and it will go directly to your title) so that it may take up a good couple of hours,while vanquishing 2,3 areas can already net u 60k+which doubled trough donation is 120k.Therefore imo storybooks are inferior.