We all live in different areas, and while most Pokémon will eventually appear everywhere, statistics says it will take some of us a little while longer to find 'em all.
Which Pokémon in the original 151 haven't you seen yet (let's just go ahead and exclude legendaries) that you're just itching to hunt down and catch?
I personally haven't seen a Farfetch'd yet. Kinda odd. Kinda want one.