I have been thinking about "key" magazines produced after 1945, to compare and contrast with the accepted canon of influential titles launched in the first half of the century; a canon that consists, by popular acclaim, of Reader's Digest, Time, Life and, er, that's it.
Clearly, popular acclaim has a large dose of Four Estate-ism in it, although RD is the token populist.
But if we accept that the postwar period, and particularly the 1960s onwards, saw massive growth in specialist magazines (pace Abrahamson, Johnson, et al) there must be some titles that were/are more influential than others. This influence does not have to be just one thing. For example, Just Seventeen represented both the apogee of publishing for teenage girls and, as it turned out, the final flowering of that genre – it reached new heights and then crashed as social changes undermined it (or "took it off at the knees" as advertising manager/publisher Zed Zawada once promised J17 would do to its rivals).
I want to try and sketch out key mags in the three main industry classes – consumer, b2b and customer/contract. So far my list, with brief rationale, looks like this:
Car: introduced great design and photography into the motoring sector, along with a literary bent to the writing
Just 17/Smash Hits: see above; massively successful teen/pop titles that were left high and dry by changing social trends
Q: successfully surfed the transition from vinyl to CD and perfected the mega-review section; moved the music sector to an older demographic
Loaded: began what become the lad mag sector
Campaign: made trade publishing glamorous and sexy
Management Today: similar to Campaign but also brought together an industry body (Institute of Management) and dynamic publisher
High Life: the in-flight magazine that some regard as the first contract mag
M&S Magazine: the first retailer magazine
More suggestions and brief rationales welcome in the comments.