Is the Northeast U.S one of the greenest places in the U.S?

YES it is one of many GREEN places in the USA. Greenest? NO

Out west the green spaces are not as crowded with humans and larger..

National Parks in the west are huge.

Near Seattle there is rain forest. That is very green.

Alaska has many HUGE green parts .The national parks in this state are about the same size as the entire state of Maine.

Vermont of New Hampshire are VERY wooded, but so is the area around Atlanta ("the city in the forest") and the PNW.

Florida and the south is also pretty green and warmer too. Plus the northeast has seen big economic decline. In the past 60 years florida, alabama, Georgia tenesee have seen big gains in the economy! 

Not in December or January.

There are plenty of places which are very green.  The Rockies and Sierras and covered with trees and vegetation (until you get too high).  The entire Northwest including northern California to the Canadian border is covered as well.  

That's a really difficult question to answer. In terms of "eco-friendliness" - another vague term - Portland OR is the greenest place in the US but whatevs. If you like the abundance of plant life, then NE US is indeed well represented. But so is the entire SE, the Pacific coast, lots of places.

If you're one of those people who suffer from seasonal allergies however, maybe "greenest" in terms of abundance of plant life isn't really what you want so much. Maybe you'd be better served by the sere beauty of the southwest desert regions, or the Black Hills of the Dakotas.

PART of the Northeast is.

Part is quite heavily-settled, many cities relatively close together.

Northern NY (the northern "peninsula"), Vermont and Maine are still fairly "green".

Good indicator

https://lighttrends.lightpollutionmap.info/#zoom=5...

Anytime I want, I can walk (For free!) in the Niagara Gorge, enjoying not only the trees and rock formations, but also the Niagara River and the wonder of the three falls of Niagara.