• R/B
• G/S
• R/S
• D/P
• Strategy
• Moves

Pikachu

This set is a good idea to lead with, considering Pikachu's decent Speed, plus the fact that if it starts, you don't have to switch in and potentially take a hit. If you predict a Ground switch, waste it with Surf, and if they try a Grass-type, Hidden Power Fire will handle them (Hidden Power Fire can also OHKO the Spiking Forretress). If they send in a special wall, you've got a chance at putting them out of commission with Sing.

Other Options

Thunder can be a gamble, but if you're using Pikachu you're probably a risk-taker anyway, and it hits hard with Light Ball. Thunder Wave gives paralysis support, which is never a bad thing, and status is about the only thing Pikachu can hope for against Blissey, Raikou, and Snorlax. Charm can hit Snorlax when it comes in to soften the blow it deals to whatever you decide to switch in. Hidden Power Grass can work if you're really paranoid about Quagsire.

Opinion

GSC Pikachu is a high-risk, high-reward Pokémon and is actually not too bad of a choice for an underused team. The addition of Light Ball gives Pikachu potential to hit things very hard, and it has a decent enough Speed stat (278, which is slightly above the "average" threshold of 268) to where it could potentially strike twice without getting hurt (once on the switch, the other right after). With a Light Ball, Pikachu's Special Attack is the second highest in GSC, after only Mewtwo. The flip side, of course, is the fact that Pikachu is OHKOed by most decent STAB attacks, so only switch Pikachu in when you know it is not going to get hurt.

Counters

Pikachu's main counter is good prediction as far as switching into attacks go. Since Pikachu usually cannot take a single strong STAB hit, if something switches in that isn't in KO range after the initial hit, then Pikachu will have to switch out. However, Snorlax and Blissey in standard will wall Pikachu all the time. Exeggutor can absorb Thunder and Surf with no problem, but can get hit hard with Hidden Power. Ground-types can naturally switch into Thunderbolt, but be wary—if the Ground-type switches into a Surf, it can probably be KOed the next turn. If you're facing a UU team, your most reliable counter is Quagsire, who can take all three of Pikachu's attacks and KO it with Earthquake. Muk is also somewhat of a reliable counter with the high HP and Special Defense (and also a powerful STAB Sludge Bomb). Another UU counter is Venusaur, who can take Thunderbolt and Surf with no problem, sport Light Screen, and recover damage away with Synthesis.