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

Parasect

Obviously, Spore is the only reason you would use Parasect. From there, you probably want to be as unpredictable as possible. Stun Spore goes with Slash and Swords Dance with Body Slam; Swords Dance is not recommended with Slash because in RBY, critical hits ignore all of the attacker's boosts, making Swords Dance futile. Mega Drain is for STAB and to hit Ground, Rock, and Water-types hard.

Other options

Hyper Beam can work with Swords Dance, and Growth can go along with Mega Drain. Otherwise, there's nothing interesting.

Opinion

Parasect isn't really good, but it might be fun to use if you know what you're doing. It's just too slow to do much, but if you can paralyze one Pokémon and put another to sleep you've already done a good job.

Counters

As long as Parasect isn't packing Stun Spore, Gengar has nothing to fear from the overgrown mushroom. Likewise, Moltres can use it to set up Agility for a Fire Spin sweep. Parasect is unbelievably frail when it comes to Fire moves; Fire Blast from Arcanine, Charizard, or Ninetales will OHKO it. Lapras can deal huge amounts of damage with Blizzard, and Dodrio OHKOs with Drill Peck. Obviously, all counters on this list need to beware of Spore.

In UU, Aerodactyl causes serious problems as do the Fire-types previously mentioned. Fearow/Pidgeot can handle Parasect as well; not to mention general Water-types with Blizzard.