I think my vote would go to the LHC, but the ISS, Saturn V, Mars Rover, 10nm process technology are all good candidates.
- Print pictures with features measured in single-digit nanometers
- Deposit layers of photo-sensitive, conducting and non-conducting materials measured in atoms
- Remove layers of materials measured in atoms
- Create devices at these geometries that execute logic at 238,310 MIPS (Intel Core i7 5960X)
There are >250,000 scientists and engineers globally working to make this happen. I can not think of another technology getting this much focus.
And it succeeded in running the "mind of the world" for quite a few years.
On the same note i would add the internet, but i'm torn, i think(maybe mistakenly) that windows is a bigger engineering achievement - something much harder to pull off, and the internet on the other hand is brilliant in it's conceptual simplicity{ routers + links + packets + redundancy + intelligence at the edge} - although the implementation has definitely pushed many boundaries.
Semiconductors are a marvelous feat of engineering but in the time from when they were first developed until now the ability to share communication has been unparalleled allowing this to happen.
In the other hand the concrete dome on the Pantheon is quite a feat as well considering their relatively basic understanding of the principals of forces and simplistic methods of collaborative sharing.