Probably something unoriginal like my Starter (Meganium), the gift Togetic, Red Gyarados, Lugia, etc. Meganium is the only one I remember for sure.
Some other Pokémon I remember using but can't remember if they were from my first playthrough or if they even made it to the E4 include Sandshrew/Sandslash, Wigglytuff, Sentret (was definitely from my first playthrough but I think I boxed it before it even evolved), and Umbreon (though I think this one was post-game since I seem to recall it evolving at Mt. Silver). Maybe Shiny Golbat/Crobat since it seems like I found it during my first file and idk if it would've outsped my Meganium post-game (tried to run, assuming my eyes were playing tricks on me, but the game wouldn't let me).