That's the problem. I can't, because whenever I seek I always get the stupid requests! GTS works well when I offer (got Graveler, Turtwig, Chimchar, and Glameow so far, now waiting for Stunky), but never seeking. I don't have enough Pokemon to stand a good chance of meeting the few non-silly offers, and this will apply for anyone during the plot.
Firstly, at LEAST put up a Kadabra for something like a level 100 shiny Silcoon or Wurmple (or a level one mewtwo, ect.)
Then, you find something that is either A: version exclusive, B: Rare, (there is almost always a Feebas/Spiritoum trade on Wi-Fi) or C: only obtainable in Platinum. For instance, offer something like Tangela or Tangrowth up for trade, and look for other offers about trades for Tangela. It will get taken.
EDIT: Right of the analisis line, a fresh batch of ratings!
Ralts (Gardevoir) - A: A good psychic is hard to get ingame, but Gardevoir fits the bill. It is better then Alakazam ingame, due to the better ingame BST (has decent defenses), and access to the almighty Thunderbolt. Is hard to level up until it is fully evolved, though.
Ralts (Gallade) - B: Caught at the same time as Gardevoir, but much worse. Bad movepool (only moves that stick out are False swipe and Close Combat) without TMs, and while you can use Kirlea for less time, the pokemon you will face mostly use physical moves... that Gallade can't take. Ow.
Lickitung - C: Without being able to use STAB explosion, Lickylicky loses it's niche. HOWEVER, it is avalable early, and can learn a few good moves via level up. Pure normal isn't THAT bad of a type, but there are better though.
[hoo boy, here come a bunch of bad pokemon!]
Eevee (Vaporeon) - B: By far the best Eeveelution, but that isn't saying much. Vappy only gets the normal water moves and Shadow Ball, really. Still, water isn't THAT BAD of a type... it's just increadably stacked. I'll make a post later in this thread about how Vaporeon doesn't stack up to competition, read that if you disagree.
Eevee (Jolteon) - C: The Eeveelutions all have bad movepools, but Jolteon takes that to the next level: he doesn't get ANY good moves via level up! That, and his movepool (actually, movepuddle) is predominantly physical: off of 65 base attack! If it had only 10 more base attack, it would be vying for S... but it doesn't.
Eevee (Flareon) - D: Infernape, Houndour, hell, even Rapidash were avalable, and you chose FLAREON? It's best attack is Fire Fang!
Eevee (Espeon) - C: A great type, ruined by a movepuddle of Psychic and Shadow Ball. Remember, back before the Physical/Special split, Bite was it's claim to fame? All psychics get the more powerful Shadow Ball now.
Eevee (Umbreon) - D: Hits like a punch from a two year old. Has the same problems as the other Eeveelutions. Put it together, and you get... Umbreon!
Eevee (Leafeon) - D: My origanal scetch had this guy with B: because Grass is a good type in ingame D/P/Pt, and good grass types are hard to get. However, I soon reilised that it's best two moves (SD and Leaf Blade) come after level 70. No. Choose Tangrowth or Torterra (just because Chimchar is S doesn't mean you have to pick him: I bet you won't have Starly and Scyther together)
Eevee (Glaceon) - D: If it were avalable as early as the other Eeveelutions, it would be huge. Ice types are ALWAYS good to have. But alas, it is found around the same time as Mamoswine, Weavile, and Abomasnow.
Only a few more, then I'll start looking at the already tiered.