I'm super biased, because Strange is probably my favorite MCU character (him and Thor flipflop for me personally all the time), but I do think there's more to Strange than what you're saying. He has a lot of that Tony cynicism, and while he's not as "I wanna be him", people like a snarky, sarcastic character. Plus within almost all the movies he's in, there's a lot of cool, flashy powers that makes his moments super memorable and fun and makes kids want to buy the action figures.
I also kinda disagree with it being hard to rank popularity, especially since Thor and Guardians are probably on the way out. At the very least, Holland's Spider-Man is a clear #1. Twitter loves to argue about how MCU Spider-Man is a sham to the original character, and I don't really care for the Iron Man apprentice stuff either, but just from my interactions with both MCU diehards and casuals, Holland has left a strong impression. Plus ya know...he's Spider-Man.
I do agree it is kind of hard to pinpoint down a definitive ranking though, since a lot of the newbies are either wet behind the ears or haven't left an instant impression like the Guardians or Black Panther did, which is a little concerning. Maybe Sam Wilson's Cap will become a big deal (at least I'm hoping), but we are missing that iconic trio that the Infinity Saga had that gets people hyped/easier to sell toys. The plan for the new trio was probably Spider-Man/Strange/T'Challa, but obviously that's not happening. And the closest to a third member is...Captain Marvel? The guy they'll get for Wolverine? Deadpool, even though he'll probably be in his own little corner? It's definitely not as concise as it was a few years ago.