system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why is the patronus charm consistently hailed as the #1 spell?
gpt
It's called the 'best', not the 'most powerful', of spells (and, on a side note, I'd argue that the 'most powerful spell' is the one that does what you need at any given time - what use is the Imperius Curse if you fall off a mountain while hiking alone?), and there are at least two reasons for this status.

One, it's a difficult spell to master.  Most wizards never attain the wisdom and knowledge necessary to cast the Patronus charm successfully.  Being able to manifest a Patronus shows that the wizard casting it is not just of average competence but *exceptionally* skilled, and every wizard knows it, making the Patronus one of the wizarding world's status symbols.

Two, it does something no other spell can do - protect you against Dementors.  Every wizard knows that Dementors are *fucking scary* and that they will *eat your soul* if you disobey the Ministry of Magic - this is a threat the Ministry uses to force compliance with the Laws of Magic, after all.  Even if you obey all the Laws of Magic religiously, and even if you can't summon a Patronus through Azkaban's wards anyway, being able to defend yourself from Dementors implies that you're on a higher level than the lowly plebs that the Ministry pushes around. 

Think of the Patronus Charm as the '#1 spell' the same way that gold is the '#1 precious metal' among Muggles.  Is it practically useful all the time?  No.  If you're starving, you can't eat gold or a Patronus Charm.  But it's seen as valuable and special, and has had that valuable and special status for a long time, because of what it represents.