Diamond: On my first playthrough, I had a Luxray I got up to Lv. 99 I think. I can't remember what I named him, sadly. I just thought he had the coolest design ever @_@ Pretty good fighter, too, from what I remember. But that was a really long time ago.
As for SoulSilver, my most recent run I acquired a Misdreavus unexpectedly in Cliff Cave and caught her, named her Sombra first and then renamed her Lumi. That thing... is a monster! It just tears through everything, like it's unstoppable! And it levels up at the speed of light, too, I think she was about 15 levels ahead of all the others. She pretty much creamed everything in her path, made the Elite Four a breeze :D
Also in SoulSilver was Carneliana, my Typhlosion. She was the second strongest to Lumi, and since I taught her Blast Burn she 1-Hit KOed the Pokemon that I couldn't keep attacking over and over. Not to mention it took me forever to get a female Cyndaquil with an acceptable personality...