The Rakuya is the second best weapon in the game, only behind the BoM (and that's only because BoM's baddass factor is infinity), but as a function it is the best weapon in the game.
It'd normal attacks are very fast, the R2 moveset is very fast thrust attacks that melt through all the kin, and the L2 attack is excellent for both crowd control and hit-and-run against a lot of bosses like the Abhorrent Beast or even the giant fish enemies, the one handed charge attack also has absurd range, which is excellent for backstabbing, and the backstep R1 has excellent range as well. The L2 also works as a secondary more powerful follow up attack if you press it after either the slashing R1 or thrusting R2.
The weapon is very fast, works well against both individual enemies and groups, has very good range, costs low stamina, and has the best moveset in the game. But do you know what's the best thing about it? It has very high damage, with the right gems it's AR can go up to 650, if you buff the weapon that's 750, with how fast it is 750 AR will melt through everything it touches, as a comparison Ludwig's holy blade maxed out does about 850 AR + 100 from buffs, considering how slow the weapon is the Rakuya's dps can easily be more than twice as much as LHB.
EDIT: The above is assuming your level is between 100-120, if you go beyond that and have both STR and SKL at 50, then quality weapons always win. And even then, the Rakuya is still better because of its moveset and low stamina cost.