This answer is going to sound stupid, but it depends on whether you want a physical attacking Pikachu, or a Specially attacking Pikachu. Raichu (I assume you will evolve it) has the same base Attack and Special Attack, so it doesn't really matter. It does have a somewhat limited move set, however. For Modest moves like Thunderbolt, Focus Blast, and Grass Knot are probably the best options, as well as potentially Hidden Power, depending on what type it is. If this is a game with the move tutors, then Signal Beam is another option.
As for an Adamant one, you will probably want to breed it to have Volt Tackle, or give it (if this is Gen V) Wild Charge. As for other physical attacks, then it gets Brick Break and Iron Tail (again, depending on the game).
So either set can work. Modest might have more attacking options, however, but which you want will probably depend on the rest of your team.