That was another question I was asked.
I find it difficult to compare the “ratings” of early 20th century innovations to the ratings of innovations today, since it is often different audiences doing the admiring, or lack thereof.
For the most underrated innovations of the last one hundred years I might pick the insights of Alan Turing, various developments in electrical engineering including better transformers, or the nitrogen-fertilizer connection, noting that in some quarters there is already plenty of recognition for each of these, Turing in particular. But still not enough!
As for contemporary innovations, I see an underrated one as Amazon’s warehousing and shipping practices. It will mean the death of much of retail and transform our suburban physical spaces into something quite…????. Into something, in any case. I am a social networking optimist, and think it has largely beneficial effects on social mores, but I also see it as having peaked at a near-saturation point.