The Flamberge has the lowest AR of all greatswords. This is made up for by how it has a bleed. Yes, a greatsword with a bleeding effect, how sub-par, right?
In the recent DLC patch, the light attacks of Greatswords and Curved greatswords were both nerfed in speed to avoid infinite stunlocking. This means that most greatswords had their utility greatly dropped, however, this instead serves to make the Flamberge overpowered. The Flamberg's R2 is the light attack of a Curved Greatsword. However, it takes 68 poise (need confirmation) to avoid being stunned, and unlike Curved Greatswords, the speed of the blow has not been touched. This permits the Flamberge to infinitely stunlock with it's heavy attack, an attack that does 35% more damage then it's light attack, and procs bleed far quicker then it's light attack. All this in a six pound weapon that can be weilded with minimum requirements and turned into a lightning weapon.
So, higher poise breakpoint, inbetween 63 and 71, dead angling, high speed, and bleeding. The only downside to this is high stamina cost, so you'll never be trapped for more then 5 swings, but that's enough to kill you.
~You have been warned~