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I'd recommend Grass or Water types, Fire types give your team a double weakness to Ground. Grass types can deal with Water/Ground types (Swampert, Gastrodon, etc.) while Water types get type coverage (i.e. Totodile with Ice Fang, Bite, etc.) and better ranged moves (i.e. Squirtle with Bubble, Ice Beam, etc.)
Bulbasaur - Has status moves, he's the 'kill it slowly' type. Sleep Powder is good for the main storyline, but isn't as helpful post-Story, with all the Pokemon that resist sleep. Charmander - Learns Smokescreen, which is always useful, and evolves into a Fire/Flying type, removing the weakness to ground. Squirtle - Good defense, good ranged moves. Chikorita - Enjoy your terrible Normal and Grass moves. Chiko works best as a Leader since she gets Nature Gifter, which allows berries and nuts eaten by Chikorita to affect all allies (it doesn't work when a partner uses it) Cyndaquil - Also learns smokescreen, but doesn't get rid of the ground weakness. Totodile - Good type coverage with Ice Fang, Bite, and Superpower. A nice partner, IMO, Treecko - Like Pikachu, Treecko is part of the IQ Group D. Works fine with Pikachu, IMO. Torchic - Gets both fire and flying moves. Better suited as a partner for Normal, Grass and Water types. Mudkip - I herd you liek. It gets some mediocre water moves as well as some ground moves (Mud Slap, which is very useful) Turtwig - Good defense, and decent offense. Learns Curse, but I don't recommend using it, since it slows Turtwig, making it hard for him to go at the same pace as Pikachu. Chimchar - Learns Torment, which may prove useful when a Pokemon repeatedly uses a ground move on Pikachu. Learns ranged moves late, though. Piplup - Learns flying and water moves, and when evolved learns steel moves. A good Pokemon once it evolves, but that happens later in the game.
IMO, Treecko and Totodile are the best starters for Pikachu.