My first two cards were a Meowth and a Geodude. I still have the Meowth but I traded the Geodude at some point :( I think my favorites are the Meowth, my Mewtwo from my history teacher, my Reshiram EX from my friend, and a random Kabuto card that happens to be in German.
Given that most people get Pokémon cards in packs of, at least, 15, and their first is usually a whole deck, I dunno if you could really get a lot of people who remember their first card, I definitely don’t! My favourite is definitely Growlithe, but I have no recollection of what my first cards were really, it’s too long ago!