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id suggest a slowbro for ur 6th. he is a great pokemon and has a good type for ur team. lacking water and psychic. also has access to fire blast/ flamethrower. id give it ice beam, surf, psychic, fire blast/ flamethrower. giving it great type coverage and it already has good defenses so it will take hits while it dishes em out.

ilovepenguinz

Get a Lapras. I think that would be a good water type for you.

Pros:
It can learn a lot of good moves. Levelling up, it learns Ice Beam and Hydro Pump, which is always awesome.
It can learn Thunderbolt and Psychic through TM, amongst other moves.
It's also a good HM slave; Strength, Rock Smash, Surf & Waterfall.
It has great HP and Special Defense to take special hits. Defense isn't too shabby, either.

Cons:
It's slow to level up.
It's got a bad speed stat, so it will be one of your slower Pokes.
It doesn't hit that hard with middling attack and special attack.

Slowbro or Starmie would have been nice since two of your Pokemon are weak to Psychic, like WooperOnALedge sort of pointed out, but that can't be helped since they're both LeafGreen exclusives only.

I'd recommend Electabuzz or Zapdos, both found in the Power Plant (Zapdos only once.)

You can give Electabuzz Psycic and Brick Break if you don't mind wasting those TMs. Therefore it has the best moveset from all available Electric types.

Zapdos has superior stats compared to all the electric-type attackers and can learn Fly and Drill Peck, but then you'd have to replace Fearow, since it completely outclasses it.

Other Options:

Raichu is an option, but Electabuzz has better stats. Raichu can't learn Psychic, but it can learn Dig. Jolteon can, too, but it's attack stat is terrible.
Magneton is decent. It's part steel so it can resist some attacks like Normal and Grass, but Ground attacks will destroy it. It's the same as Jolteon in that it can only use Electric attacks decently, but it has great special attack.
Electrode can explode and is extremely fast, but apart from that, it has bad stats, and apart from Explosion, can only use Electric attacks.