Gate would normally be the answer, but if the planar ruler is unwilling, they would prevent this spell from opening near them:
You conjure a portal linking an unoccupied space you can see within range to a precise location on a different plane of existence. ... Deities and other planar rulers can prevent portals created by this spell from opening in their presence or anywhere within their domains. ... When you cast this spell, you can speak the name of a specific creature ... [on another plane] ... and [draw] the creature through [the portal] to the nearest unoccupied space on your side of the portal.
Wish could easily be the only answer, but it is not ideal since that would risk never casting wish again:
... You might be able to achieve something beyond the scope of the above examples. State your wish to the GM as precisely as possible. The GM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong. ... The stress of casting this spell to produce any effect other than duplicating another spell weakens you. ... Finally, there is a 33 percent chance that you are unable to cast wish ever again if you suffer this stress.
I would also rule out Planar Ally, since presumably being a planar ruler would mean that the target of the spell would be the planar ruler themself (even if "planar ruler" does not necessarily imply "being of cosmic power", assume it does for the sake of this question):
You beseech an otherworldly entity for aid. The being must be known to you: a god, a primordial, a demon prince, or some other being of cosmic power. That entity sends a celestial, an elemental, or a fiend loyal to it to aid you, making the creature appear in an unoccupied space within range. If you know a specific creature's name, you can speak that name when you cast this spell to request that creature, though you might get a different creature anyway (GM's choice).
Is there a different spell or magic item that can be used to conjure an Elemental planar ruler, specifically a noble genie?