Favorites are favorites for lots of reasons. Sometimes it's the route itself, sometimes for what happened along the way. Here are some favorites, selected to illustrate geographic diversity. (PS -- with all of these rides, you can adjust length and start/end points to serve your purposes.)
Berkshires / Mount Greylock Loop. 40 mile circumnavigation of Mount Greylock, the tallest mountain in Massachusetts. The ride around the mountain is scenic, especially the rail trail on the eastern side. This was the final ride in our 351 series. Mike summited in 2015.
Deerfield Valley: Hoosac Tunnel Loop. 30 miles taking you up into the wild near the Vermont border, following by the steep rush back down to Monroe Bridge on the river. On the river ride home, you pass the entrance to the Hoosac Tunnel, a 5 mile long railroad tunnel, one of the engineering marvels of the 19th century. Still in use today, cool and spooky from the outside.
Central Mass / Quabbin Loop. 55 miles riding the 'inner trail' to visit the drowned towns, it's 10 miles more taking a wider loop and staying on roads. Very rugged country, but very scenic, especially along the reservoir.
Merrimack Valley Ramble. 75 miles, the first 30 following the Merrimack up to New Hampshire. The ride is surprisingly scenic and uncongested, even when going through Lowell and Lawrence. I thought of the Merrimack as a polluted, toxic industrial zone. Quite the opposite. The route home incorporates both the Nashua River and Minuteman Rail Trails. Stop at Lexington Green, where the American Revolution began. Take the commuter rail north to Haverhill for the start of this ride. Finish is in Malden Center, on the orange line / commuter rail.
Cape Cod / Woods Hole. 75 miles, Hyannis to Woods Hole and back. Incorporates the very scenic Cape Cod Canal Trail. Cape riding is always ego boosting -- no hills! (just watch out for the sneaky one up Rte 130 out of Sandwich!)
Martha's Vineyard Loop. Mike's family (wife + 3 teenagers) indulged him in a family ride 43 miles around Martha's Vineyard, covering all 6 towns on the island. The first several miles from Oak Bluffs to Edgartown are along the ocean and very pretty. Watch out for 1.5 miles of 'sand road' (Old Woods Road, mile 25). My daughter doing the ride on a mountain bike was amused with our road bike struggles on that stretch.