i have not got diamond or a DS yet( will for my b-day, though), so i'll probably go with Chimchar.
All 3, assuming the system me and my friend will use works....
Is it possible to have 2 games (mine and my friends), and pick a starter, advance until you can trade, trade to my friend, and repeat until you have all three starters?
You don't need the Pal Pad for local trading, so you can just go until the second town and trade. It's what my roommate and I did.
I started out with Piplup the first day, but I quickly deleted that file. When you're at the Elite Four, and you have a Level 42 Empoleon and a Level 34 Staraptor, you know you're doomed to die.
Now I use Chimchar because it can beat the most Gyms easily.