I really want a Surfing Pikachu, but what Pokemon should I use?
According to IGN:
To teach your Pikachu Surf (a technique it can normally not learn), you need to first unlock the R-2 (Round 2) mode. Then do the following:
- Enter the R-2 Prime Cup's Master Ball division (you have to beat the first three divisions first).
- Assemble a team that contains a Pikachu that does not yet know Surf. You HAVE to use a Pikachu from your Game Boy game. You cannot unlock Surfing Pikachu using a Rental Pokemon.
- Do not register your team! Pick it directly from your Game Boy cartridge.
- You now have to beat the Prime Cup's Master Ball division using a three-Pokemon team that always contains Pikachu. Pikachu has to be in every one of the eight battles.
- You are not allowed to use any continues. If you do, you won't get Surfing Pikachu.
Once you have fulfilled all the above conditions and beaten the cup, you are shown an award screen with Pikachu standing on a surfboard. From here on, Pikachu can use the Surf technique both in battle and during gameplay (Game Boy versions only). If you've got Pokemon Yellow, you can now also access the hidden Surfing Pikachu mini-game. Simply go to the small hut on the northern beach of Route 19.
So which pokemon you want to use is up to you. As long as you use a Pikachu you caught for the Master Ball division, you're golden.
You're going to want an assortment of elements, I'd go with the following (all considered OU in gen 1 except mewtwo)
Cloyster (or lapras)
Rhydon (or golem)
Chansey (or snorlax)
Also have a few coverage moves, like ice beam or surf on dragonite and thunderbolt on chansey. Since almost everyone on the list can learn ice beam, I'd go with a clamp build on cloyster like smogon says.