If you want to do the most work for the smallest climb then West Bond is calling your name. A 22 mile out and back for 4,500 feet.
For a concentration of peaks rated Very Difficult then you need to hit up Baxter Park, where there are three challenging summits.
The quickest way to get 4000 feet are Saddleback Mountain in Maine which is a 3.8 mile trip and Cannon Mountain in New Hampshire which is 4 miles (and you could even take the ski lift down in the summer)
I'm a peak bagger through and through. I don't know if I would ever get off the couch for a walk that didn't involve a summit finish with a view. For that reason I put together this dashboard that concisely displays all the 4000 foot peaks in Maine, New Hampshire and Vermont. Hopefully this can help some of you plan that next ascent!