What Is The Most Popular Tree In The World?

What do you mean by "popular"?

Popular in domestic gardens, or the favourite for timber extraction?  Favourite for making things with (like furniture), or most chosen when landscaping in public areas?  Or just the one that the public would choose as "prettiest" if surveyed?

The problem is that trees are living things, with their own peculiar  requirements.  They have different needs with regard to sunshine, water, and soil chemistry and associated funghi.  So the most commonly planted tree in southern California is different from the favourite in Europe, or India, because what can grow depends on climate.

If you ask people what trees they like the look of, they will choose things like Maple, Redwoods, Chestnut, English Oaks, and old-growth conifers.

In the USA, Cherry trees dominate gardens in wetter places, citrus in the gardens of deserts and coastal California.

Conifers are the most popular trees in commercial plantations in temperate climates, world wide.  Which evergreen depends on local conditions, but pines and Sitka  Spruce dominate.  Larch and firs take over as you head into colder climates.

In warmer and drier places (Southern California, India, Australia) Eucalyptus dominates.  Either Pines or Eucalyptus are the most commonly planted trees in the world.