It is hard to pick one, so I am gonna go with 2.
If my signature pokemon was a starter, it would 100% be Empoleon. Love penguins and swords and it is the perfect blend of the two. Empoleon is also my favorite pokemon of all time.
However if it wasn't a starter pokemon, it would probably be Normal Rapidash as it has a very special place being my first Shiny and also a horse being very famous where I live.
As a runner up to these two, Braviary would probably be close, and even though I love eagles, it doesn't scream signature to me. But it would be used a lot.