I always liked Porygon, ever since Red/Blue, but they were so incredibly hard to get. I got one in the end, but never battled with it. When Porygon-2 came along, I loved it, but again I couldn't get it, this time because I had no-one to trade it with.
Finally, DS wi-fi connection and Porygon-Z come out. I could finally have a form of Porygon on my team without a ridiculous amount of work.
The 10 year wait was worth it for me. It's a good Special Sweeper, and Conversion+Adaptability is godly. I taught mine Solarbeam, Thunderbolt, Recover and Conversion, meaning when it uses conversion to become either Electric or Grass, Solarbeam or Thunderbolts power is doubled. That coupled with a great Sp. Att. stat means it's pretty damn awesome.