2. Giovanni's Mewtwo
During the first season, Pokemon made an amazing episode for the backdrop of Pokemon: The First Movie. The episode was surrounding Ash having to go back to Viridian City to obtain his last Gym badge, but before he takes to the stage, we see Gary battling it out with Giovanni (The Gym Leader). This is where we catch our first glimpses of Mewtwo, at this time under the control of Team Rocket and Giovanni is using him to annihilate anyone that comes before him, Gary being one of them. Luckily, Ash catches a break and when Giovanni is unable to supervise the Gym, he leaves it in the bumbling hands of Jessie & James, who as you would expect lose. It was a smartly devised storyline as we couldn't see Mewtwo at the time, as he was wearing a suit of sorts, so it still left a mystery to what Pokemon it as, but at the time it left its audience puzzled to find out who and what is this incredibly powerful Pokemon.