In pursuit of this, y'all should be aware that the current Global Mission is "trade as many pokemon over the GTS as you can!" Which means people are putting up reasonable offers instead of just trying to trade a jigglypuff for an arceus. If you have some reasonably good stuff, or duplicate legendaries, head over and see what you can get.
I got Torterra easily (the final non-legendary I needed for my dex), then got Solgaleo, then duplicates of cosmog and Type:Null to evolve.
Someone also traded me a Buzzwolle for random klefki that wandered into my PokePelago.