First pokemon I got to 100 on Diamond was my Pichu.
First on Pearl was my Milotic.
And on Platinum I got four level 99s and one level 92, I'm not sure which I want to 100 first
On red was my Zapdos.
Blue it was Oddish (I KNOW LOL IT WAS THE FIRST GAME I HAD).
On yellow it was Pikachu.
On Gold/Silver/Crystal I got my Feraligator to level 100 on all of them.
On Ruby/Emerald I got my Metang (i forgot its name ;_; ) to 100.
And on Sapphire it was Kyogre.