Parasect isn't really good, but it can be fun to use if you know what you're doing. It's just too slow to do much, but if it can paralyze one Pokemon and put another to sleep it's already done a good job.
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 a waste of time. Mega Drain is for STAB and to hit Ground-, Rock-, and Water-types hard.
Hyper Beam can work with Swords Dance, and Growth can go along with Mega Drain. Otherwise, there's nothing interesting.
Checks and Counters
As long as Parasect isn't packing Stun Spore, Gengar has nothing to fear from it. Likewise, Moltres can use it to set up Agility for a Fire Spin sweep. Parasect is unbelievably frail when it comes to Fire-type moves; Fire Blast from Arcanine, Charizard, or Ninetales will OHKO it. Lapras can deal huge amounts of damage with Blizzard, and Dodrio OHKOes 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 and Pidgeot can handle Parasect as well, not to mention most Water-types with Blizzard.