Mt. Tom: December 1988?
Although I don't get out as much as HikerEd (who does?), I was out there before him and my first two trips happened to bracket the major event associated with the now-excellent views from Mt. Tom.
In July 1987, I was just beginning my quest for the 4000-footers and couldn't resist the lure of a three-fer trip to Field, Willey and Tom. At that time, Field had excellent 180-degree views of the Pemi (and the trail headed straight for Willey from the summit instead of the sideways start of today). When we got to the start of the Mt. Tom Spur, half the group stayed there not wanting to bother with the extra 0.6 mile each way. I soon learned why: the summit of Mt. Tom was as wooded and viewless as Owl's Head; this one was strictly for the peakbaggers.
In October of 1989, I came back with a friend who had joined me a bit late on my 4Ks and since I was done, I went back to these three with him. Imagine my surprise to find hordes of people ascending and descending the Mt. Tom Spur. Sure it was Columbus Day weekend, but still. I had warned my friend that there was nothing to see up there and was stunned to find that the summit now boasted one of the best views of the range (it would unfortunately be years before I discovered - in summer anyway - the complementary eastern views from just below the summit).
While it does appear to be getting even better over time, most of the improvement happened between July 1987 and October 1989. My inquiries at the time were answered by talk of a December 1988 storm but I can't offer solid evidence for that particular event.