The largest terrestrial biome on earth is the taiga. This is defined as being a forest that has a lot of evergreens and coniferous trees. These don’t lose their leaves over the winter. There are many pockets of this kind of territory over the entire world.
I mean, there are small wooded areas near my home that have trees that don’t lose their leaves this time of year. It’s absolutely beautiful and breathtaking to see a line of pine or evergreen trees on a snowy day. It’s followed closely by the desert, then the deciduous forest, then the tundra.
However, if we’re just talking what the largest biome on earth is - terrestrial or not - it’s the marine biome. That covers 75% of the world, since three-quarters of the earth are covered in water. That’s really all there is to that.