Who was the first alien to land on earth?

No verifiable evidence has ever been presented to indicate that any alien has ever landed on earth.

However, complex organic molecules have been shown to exist in meteorites, so some of the origins of organic life could be called "alien". It has also been suggested that life itself was brought to earth by asteroids or meteorites (see exogenesis and panspermia), making all life inherently "alien".

These hypotheses have no evidence backing them, of course, and are not really useful. It really doesn't answer the question of how life began, it just kicks the question to another planet, without addressing how it came about *there*. An explanation that doesn't explain is not valuable to science. In science, the simplest explanation that fits the facts is preferred. The simplest explanation for life on Earth is that it began here on Earth.

So in answer to your question... we are still waiting.