aguy777:
...I know that can't be 100% true, as any Shadowrun game gets more applicants than they can shake a stick at...
Well, there, you're getting into a semantic difference between 'most common' and 'most popular'...
There are a lot of really popular older systems out there that tend to get a big response every time they open up for new players, just because people really enjoyed playing them before or they have some sentimental value, or even just because of name recognition ("I keep hearing my friends talk about this game...I should try it out...")
That said, because they're older systems/editions, they're out-of-print and some may or may not have PDF versions available, so while there are no shortage of people who are interested in them, there's not a whole lot of NEW interest in them, so they become less and less common.
Even the discussion about D&D being the most common game out there (which is almost certainly a given, since it's probably the single-longest RPG in production and without a doubt one of the best marketed) becomes a little dicey, because it's a REALLY broad category and while the base mechanics are still pretty much the same, AD&D was vastly different from 3E which was vastly different from 5E and there's also a small forest of mostly-homebrewed games that people are listing as 'D&D' because they're using those same basic mechanics with a bunch of their own home-rules--but it's still closest to D&D so people will recognize that listing.
And, as stated, games tend to come in waves. Granted, I haven't looked for a new game for a long, LONG time, now, as I'm generally content with the ones I'm in and what little gaming time I have beyond that is taken up with games I've been invited to join...but I can remember, at various points in the past, seeing Wanted:Players filled up with ads for Serenity/Firefly games, Star Trek games...Predator-themed games had a heyday for a while...Harry Potter...all in addition to the standard leaders of D&D, Shadowrun, Pathfinder, etc... A little over a decade ago, I could find, on any given day, five or six games that were running the D6 Star Wars rules, but I don't know if there are any that are even active, anymore, much less looking for players.
So, the answer to your question is highly conditional and could be very different a month from now.