In-game:
Thunderbolt. 90 BP and STAB, and being 100% accurate, should be enough to cover any Water or Flying types you meet. In-game, you pretty much want to brute force everything, so having your main attack constantly miss is pretty bad.
Competitive:
It really depends on your team. If your Pikachu is alone without any support, use Thunderbolt again. Unless you're using Raichu, Pikachu will probably be holding a Light Ball, which means you can't use any other items that might boost your accuracy or anything.
If you're using Pikachu as a Rain sweeper however, Thunder is definitely the way to go. It has 100% accuracy in rain.
