A "table" has a "tabletop."
A "counter" has a "countertop."
A "desk" has a "desktop."
Each is, exactly as its name suggests, the "top" of the object, the big flat slab-like top piece that is the working surface.
Usually, people just call it "the counter."
Only when you are remodeling a kitchen, you distinguish between the "countertop" and the "counter." Often the countertop is supported by a cabinet with drawers, and often people will decide to keep the cabinet but replace the countertop.
If you were admiring somebody's kitchen, and it was the beautiful granite countertop that caught your eye, you might say "what a beautiful countertop" because it is the countertop specifically that you like.