Since you have the choice as "Raichu" and not "Light Ball" Pikachu, I'd go with Ampharos, and here are a couple reasons why:
~ Ampharos is much more balanced than Raichu. Raichu can go ahead and hit the opponent pretty hard, but it dies just as it easily. Ampharos is a bit more durable than Raichu, although its not as strong. However, for playing through the game, bulk is better.
~ Ampharos has a much better movepool to suit it. Raichu has some cool moves to learn, but none really benefit it besides Focus Blast, Thunderbolt, Brick Break, Iron Tail, etc. Ampharos gets hands on Thunderbolt/punch, Fire Punch, Brick Break, Iron Tail, Power Gem, Signal Beam, etc. Ampharos has WAY more options does.
~Ampharos has a higher Experience base, but is one that is more suited for the earlier levels. Pokemon with Ampharos' base only get hard to level at around level 60; past the halfway mark. While this may not be as great as the others, if you're a casual Pokemon gamer, then a lower initial EXP base is what you want. Less Experience to gain = faster leveling.