Hardest question to me, as Gen 3 is my favorite, and I love so many pokemon from it. I'd choose one of either (yes this is turning into a list but for the love of everything I can't decide):
-Absol: Not just my favorite pokemon of all time, ever, but also useful in that can predict disasters to keep me safe, it's beautiful and it carries a sweet little punch to its attacks.
-Swampert: Insane HM utility means I can go whereever I want, its typing means it can beat almost anyone it wants (screw Wallace's Ludicolo), and it's bulky.
-Metagross: Nobody said Birch can't have any of those. It levitates, helps me go anywhere, Steel typing is OP in Gen 3, shiny Metagross is beautiful, thing hits like a truck