Ask an old timer from southern New England

"Which railroad had the biggest steam engines in New England"

The answer you get will often depend on where you're standing, if you are in Connecticut, Rhode Island, or southern Massachusetts, you will probably hear "New Haven 3300 series. 

You may hear New Haven 3500 seires

New Haven served Boston, Hartford, Springfield and New York City. But their power wasn't the biggest

Standing in Southern Massachusetts you may hear New York Central's Boston and Albany Berkshires

Powerful High speed power for crossing their name sake Mountains, but No.

North of Boston and along the top of Massachusetts you may hear Boston and Maine's Mountains were the biggest.

But again, it would be wrong.

The Largest Steam engines to operate in New England Belonged to the one railroad that regularly ran smaller engines, and managed to run from the Canadian Border, south through Vermont, Massachusetts and Connecticut without passing through any major cities, The Central Vermont Railroad.

The 2-10-4, 700 series, Texas type.

The reason most southern new Englander's wouldn't even think of the central Vermont is that below Brattleboro Vermont the railroad used mainly small Consolidations. 

The Bridges below Brattleboro couldn't handle the weight of the 700's

As far as for the classiest looking engines in New England?
Baldwin Hudsons, New York New Haven and Hartford Railroad I-5's.

But that's just my opinion