In pearl, I've always had two: Luxray and Flaaffy. I had my Luxray since he was lvl. 2, and it's never actually fainted. It's never been out of my team, and I've become stuck to it.
Flaaffy, my favourite pokemon. Never been out of my sight since I hatched the mareep egg I got in a spin trade. Hatched her in victory road, gave her an experience share and now she's my strongest pokemon. After she evolved from a Mareep, I made sure that she had an everstone.
For HG, Meganium, Leafeon and yet another Flaaffy.
Meganium was my starter pokemon. He was suprisingly strong, even then, and I was pretty suprised - to me, he seemed to learn moves so fast. I made sure I evolved him, as I love Meganium, and it's been the same since
This flaaffy has pretty much the same backstory as the other one: Spin trades, experience shares, everstones, all of it.