I'm not sure how one would go about compiling an objective list of the top 10 or 20 or 30 mathematicians (which would be required to answer your second question), but to answer your first question - we probably keep hearing about him because he was undeniably a good (if not great) mathematician and that his story is both inspiring (or feel-good) and tragic at the same time. We like narratives, and his life story is a narrative that is almost "magical" (to quote Steven Wolfram's blog post on him, "Will there ever be another Ramanujan? I don’t know if it’s the legend of Ramanujan or just a natural feature of the way the world is set up, but for at least 30 years I’ve received a steady stream of letters that read a bit like the one Hardy got from Ramanujan back in 1913 ... Are these numerical facts significant? I don’t know. Wolfram|Alpha can certainly generate lots of similar facts, but without Ramanujan-like insight, it’s hard to tell which, if any, are significant." [1])

[1] http://blog.stephenwolfram.com/2016/04/who-was-ramanujan/

His story is very appealing as a story: young mathematical genius from India, learns mostly as an autodidact from only a few resources, re-discovering things on his own, often poor and sick. Finally gets recognized as a "genius" by the western mathematics world. Then dies young, leaving many insightful works that are only understood and applied years after his death.

And while it's incredibly hard to come up with a definition of "greatest mathematicians", he is quite high up there in many rankings, top 30 at least.

I am not a mathematician, therefore, thoroughly underqualified to judge mathematicians. However, I recommend George Andrews' lecture[1] on Ramanujan's relevance to current mathematics. I wish they'd make more movies about (other) mathematicians and scientists too, minus the fake drama like Ramanujan's physical abuse seen in this one.

[1] https://www.youtube.com/watch?v=y_0NuOBNobk