Definitely between 1950 and 1952.
1950 was the year Lionel really advertised Magne-Traction to the fullest extent, and some great illustrations of the then new 773 Hudsons and the 2023 UP ALCO's.
1952 though definitely has the best catalog cover and the best two page accessory spread with the 356 Freight Station, 362 Barrel Loader, 445 Switch Tower, and the 450 Signal Bridge.