Diamond: Naetle
Pearl: Pochama (most probably)
I like all three starters, so I've made my decisions based on the other pokemon I want to use. I try not to use two pokemon of the same type if i can.
In Diamond I have other ideas for a water and fighting type, leaving me with Naetle, although I will be lacking a fire type.
Pearl, I have another idea for a grass type. I need a water type (Surf etc.) in-game so will go with Pochama.
So both teams will be lacking a fire type (I could use Ponyta but I ususally use 100% new pokemon on my first couple of play throughs).
I really like Hikozaru though...