As you should know, there is Kurzick faction, Luxon faction, and Balthazar faction.

In my opinion, either Jade Quarry(JQ) or Fort Aspenwood(FA) are the best for gathering faction, because either of those two allow both an alliance faction and Balthazar faction to be gathered simultaneously.

If you want Kurzick/Balthazar faction, go to JQ, the Kurzicks dominate there.

If you want Luxon/Balthazar faction, go to FA, the Luxons dominate there.

However, if you just want alliance faction, there is an even faster way, although it's also possibly harder. You will need a good team capable of mass damage and mass healing to make this an easy and thus worthwhile run.

Fast Farming Kurzick: Just outside of Altrumn Ruins in Hard Mode, farm all the wardens and kill the four bosses. A good team can accumulate approximately 1400 points in under 3 minutes.

Also, a good team can vanquish Drazach Thicket in under 25-30 minutes, accumulating approximately 17,000 Kurzick faction.

Fast Farming Luxon: The Jade Arena(a repeatable quest found in the Jade Sea just outside of Breaker Hallow) can be completed in under 2 minutes and provide 400 Luxon faction if completed by leaving Jade Flats instead. If you leave from the usual point, Breaker Hallow, it can take about 3 minutes.

Also, a good team can vanquish Mount Qinkui and accumulate about 11,000 faction in 6-9 minutes.

Repeatable Quests include: Duel of the Houses(Kurzick), The Jade Arena(Luxon), In the Army Now(Kurzick), In the Army(Luxon), Kurzick Supply Lines, Luxon Supply Lines, Scouting Ferndale(Kurzick), Souting Maishang Hills(Luxon), Securing Echovald Forest(Kurzick), and Scout the Coast(Luxon).

For simple farming, it's much slower, but just make a good build for your character and style then accept the blessing from a Kurzick or Luxon Priest located at resurrection shrines throughout the Luxon/Kurzick territories. The effectiveness of this method changes according to the location of the Kurzick/Luxon border, but generally you will need healing spells, AoE spells, a tank, and someone capable of pulling. My advice is at least two monks(healers), two warriors(tanks), an elementalist(nuker), a mesmer(spiker or shutdown), a ranger(puller), and a necromancer(minion master).