Pokemon crystal what Pokemon do you have to trade to get?

Many Pokemon will evolve through trade. Four of them don't require a hold item, and the rest do. Kadabra evolves into Alakazam, Graveler evolves into Golem, Machoke evolves into Machamp, and Haunter evolves into Gengar when you trade. The rest are down here, or at least what I can think of:

Need to hold Metal Coat:
  • Onix > Steelix
  • Scyther > Scizor

Need to hold King's Rock:
  • Slowpoke > Slowking
  • Poliwhirl > Politoed

Need to hold Dragon Scale:
  • Seadra > Kingdra

Need to hold UpGrade:
  • Porygon > Porygon2

There are also Pokemon not obtainable in Crystal itself that must be traded from other versions to get. They are:
  • Bulbasaur, Ivysaur, and Venusaur (Trade from Red, Blue, or Yellow)
  • Charmander, Charmeleon, and Charizard (Trade from Red, Blue, or Yellow)
  • Squirtle, Wartortle, and Blastoise (Trade from Red, Blue, or Yellow)
  • Vulpix and Ninetales (Trade from Blue, Yellow, or Silver)
  • Mankey and Primeape (Trade from Red, Yellow, or Gold)
  • Omanyte and Omastar (Trade from Red, Blue, or Yellow)
  • Kabuto and Kabutops (Trade from Red, Blue, or Yellow)
  • Articuno, Zapdos, and Moltres (Trade from Red, Blue, or Yellow)
  • Mewtwo (Trade from Red, Blue or Yellow)
  • Mareep, Flaafy, and Ampharos (Trade from Gold or Silver)
  • Remoraid and Octillery (Trade from Gold or Silver)

Keep in mind that in Red and Blue, you choose from only one of Bulbasaur, Charmander, and Squirtle, so you would need three of any of these games. However, in Yellow you can get all three, so it's probably the best choice. You also get a choice between Omanyte and Kabuto in all three games, so that will require you to get another. Mew and Celebi are only available via Gameshark, unfortunately, so you'll have to make do without them unless you're okay with hacking your game.

In order to get all Pokemon, I would reccommend getting a Yellow version, and then either a Red or Blue version, (Or both!). You'll also need either Gold or Silver.