For a casual playthrough, good natures aren't required. You can do well with a really bad nature in-game. With that being said, for both of them, either Timid or Modest is recommended. It would be better to evolve Pikachu into Raichu to get full benefit from it. The reasoning behind both of these natures is simply based on their best stats, and the fact that in-game playthroughs are usually more based on who can hit faster and stronger, and less based on bulk. Modest is usually preferred over Timid because usually both of these Pokemon would outspeed everything anyway. The reason I'm not going with Lonely here is because both of these Pokemon have their STAB (Flamethrower/Fire Blast/Fire Punch??? for Charizard, Thunderbolt/Thunder/Thunder Punch??? for Raichu) be Special due to the physical-special split not happening yet. If you really don't want the attack drop from Timid or Modest, you can go for Hasty, Mild, Rash or Naive for either of them, but Modest or Timid is advised.
Hope this helped.
Edit: As sumwun said, Lonely/Naive might be better for Charizard because of Fly/Earthquake, which is valid. Assuming you want to use the Earthquake TM on Charizard, you can use either Lonely/Naive.
