I don't have any experiences with the other generations and I am only starting out to play 4th gen (Pokemon Platinum) after my save from White got erased by my nephews. I'll just share the story of my best friend whose an avid pokemon player since the metal generations (R/B/Y/G/S/C).
My friend finally poketransferred his favorite pokemon of all time by borrowing my DS, Kabutops. When I saw the summary screen of it, I saw "Kanto" and the phrase "After a long journey through time". I wondered where the hell did it came from cause I only encounter pokemons from Sinnoh, Hoenn, Johto. He said his Kabutops came from his Pokemon Red game during the Gameboy days.
He had this nostalgic look in his eyes everytime he looks at his Kabutops. I wonder how many years had passed since Pokemon Red to Pokemon Black? :s
That is impossible, it probably came from the 3rd Gen games, since that's the earliest that it can transfer up to. (If that made sense)
Anyway, on my Japanese Black, I'd say it's my Level 6 Squirtle. :3 First shiny hatched on my Leaf Green version, since then he's been transfered over, and hasn't been leveled up... well except that one time when he grew to Level 6.
About 4 Years and it's still around at level 6 is quite good I think. :3
Same with me I am afraid.Sadly don't have another DS to transfer from DPPtHGSS to Black, if I did, I'd be sending over my Metagross from Ruby that is sitting on my Pearl game.