my most memerable, after searching on the GTS in diamond for 2 months trying to get a palkia, i got on early in the morning to see if i could find one and then, a miracle! someone was so desperate for the spiritomb i had up for trade that they traded a palkia for it! i searched for a palkia anyway since i might get a better one and found someone that wanted an eevee for it! later they both passed the legality check!
my least memerable trade... that would have to be when i traded my legit shiny pachirisu for a shiny scolipede only to find the scolipede was severally hacked, but later i was able to trade it for a salamence so it worked out