Poplars aren't aspens, and the poplar election is over anyway. One of my favorite areas is the Aquarius Plateau in Utah, between the Escalante River and Capitol Reef.
The eastern Sierra is closer for me, however, and always has a good show. But there's some backcountry groves on the western slope of the Sierra that have by far the largest and proabably oldest aspen trees anywhere I'm aware of - up the 3ft diameter. (Never mind that aspen groves regenerate from a common root system
that can last thousands of years, prompting some scientists to classify them as the
world's oldest trees - I'll stick with the layman's definition of a tree! But this is the
reason why different groves adjacent to one another will turn different colors at the
same time of the season.)