Though I initially was thinking of this topic with regards the GTS, I thought I may as well branch it out to all the games. Here, we all recall and celebrate the most spectacularly one-sided trades we've ever made. Mine will all be GTS related, 'coz it seems like it's easy to rip people off there.

How long has it been since I've actually caught a Pokemon? It's been a fair while actually; I pretty much stopped, far more content to pimp out a couple over the GTS for whatever I please.
Here's a tip for anyone who wants to use the GTS. Breed lots of Eevees. Lots and lots and lots. Why? Because they're still as wildly popular as ever, and some people don't seem to realise you can get one for free after you've gotten the National 'dex. I've had a bit of success with Murkrow and Elekid as well, but trust me, Eevee is the best I've found for this.

To first test this theory, I offered a freshly hatched female Eevee in exchange for a Spiritomb, since I've never managed to catch anyone underground. I left it for a couple days, came back, and bam, lv.25 Spiritomb, caught with a Master Ball, had been sent my way. So I went for the big one, and in a move I still can't quite fathom, I made the most one-sided trade I'd ever managed.

Eevee for Dialga. No way that would work, right?
...Wrong. MALE Eevee, newly hatched, sent off for a week. And when I came back, there it was. A Master Ball-dwelling Dialga, adamant orb attached, at level 100. So can anyone explain this to me, other than people really love Eevees?

And feel free to see if you can top that little swindle.