I usually like to send something nice that isn't on the usual Kalos PokeDex, like for instance I sent out a box full of Tropius this morning. Snagged me a Cottonee.
UNOBTAINABLE, THEY SAID.
When I was first using the feature I did immediately throw crap into it, but my moral calculators force me to be really nice and spend some time to hatch up something that people would actually like. Because even if they sent you crap, you gave them a rare pokemon without the hassle of searching through the "I WANT ZAPODS" GTS pages- and all's the better (and more likely) that that Pokemon landed on their team or is the ancestor to perfect 6IV pokemon. Sending a poor Bunnelby will only amount to it being hated, sent back, or released.
I know that the ridiculously-overpowered-compared-to-the-rest-of-my-team Arcanine I have now was from some girl named Charlotte Wonder Trading newly hatched Growlithes, so I aspire to be that guy always sending good stuff through. You just might make several people's day.