(assuming you can trade only when you're evolving a Slowking or Scizor)
In general, Pokemon that have a higher attack stat are better than Pokemon that don't, which is why I think Gyarados is better than Slowking. Additionally, you can find a level 50 Magikarp on Route 43, which is better than both Slowking and the red Gyarados.
Heracross is usually better than Scizor because the metal coat takes a long time to get and fighting is a better offensive type than steel. In the entire Johto Pokedex, there are exactly 5 Pokemon that are weak to steel and not weak to fighting.
You also have the option of not using any of these and just going with the 4 Pokemon you already have. Using less Pokemon means each Pokemon gets more experience and reaches higher levels.