Atlas, K-line, Lionel, Marx, MTH, Weaver and Williams have continued to manufacture some great O gauge product over the years; it is easy to see if someone becomes brand loyal to just one producer of toy trains.
As for me, it would be Lionel. I had those under the tree at Christmas time; it is one way of mentally keeping a youthful outlook as I grow older with time. There is no stray cat in me when regarding toy trains.
enjoy your trains...