It can be, but that doesn't seem like a reasonable interpretation here at all. There can be (and is) a category of "most important" drug breakthroughs, which is

It can be, but that doesn't seem like a reasonable interpretation here at all. There can be (and is) a category of "most important" drug breakthroughs, which is one step above "important", two steps above "notable" etc. Ranking the breakthroughs within their category may be possible but would require a lot of arguing and term definitions, so instead we just say "among the most important".