Snorlax & the Poke Flute

Snorlax is the best all-round Pokemon in Generation I - 160 base HP, 110 base Attack, Body Slam, Rest, Earthquake and Hyper Beam. You get at most two chances at one.

Step 1: Clear the Pokemon Tower

You need the Silph Scope from Giovanni in the Rocket Hideout to get past the ghost on 6F - which turns out to be a level 30 Marowak that cannot be caught.

Step 2: Rescue Mr. Fuji

He is at the top of the tower. He gives you the Poke Flute.

Step 3: SAVE before you play it

There are exactly two Snorlax in the game, one on Route 12 and one on Route 16, both level 30. Knock one out by accident and it is gone forever. Save first, every time.

Step 4: Set up the catch

Put it to sleep or paralyse it, chip it down to a sliver with a move you control - not a big Normal attack - and throw Ultra Balls. 160 base HP means you have room to work.

Step 5: Use the Flute in battle too

The Poke Flute also wakes your own sleeping Pokemon mid-battle, which matters against Koga and Agatha.