Bow of the Clever actually works disturbingly well with Ignis' Overwhelm technique. I don't know why, but it essentially just makes Noctis keep shooting at the target (it shoots pretty fast as well) until it's either dead, you've run out of HP, or... I dunno, until it just gets bored of you firing so many shots. But yea, due to that, unless I'm missing something, that little combo inadvertently sort of makes it the best Royal Arm, or at least Top 3.
I also like the Shuriken. Heavily disappointed that it got nerfed from how it was in Platinum Demo (The one good thing about the demo no less), but it's still a decent mid-range weapon for those monsters you really don't want to get close to. Sword of the Father gives the most clear-cut passive boost, so I guess it's technically the best, not to mention it's pretty neat owning your father's sword. I also like the Blade of the Mystic, the Shield of the Just, and the Greatsword of the Tall.