MissingNo was the best since I could get all my Pokemon up to level 100 easily. My favorite though was the Mew glitch in Red/Blue. I found a guide for it online and it seemed like it was made up because the steps it had me go through were so random but I wanted a Mew so figured I’d try it.
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On route 8 go to where the gambler is on the west side and set up your character to where if you go down one step you’ll enter into a trainer battle. Press down and immediately press start and use a Pokemon to fly to Cerulean City. The gambler will then have the “!” pop up above his head but before he starts moving your character flies away.
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Travel north to Route 24 and battle the youngster whose only Pokemon is a Level 17 Slowpoke. This requires that you have skipped this character when going through this route previously and it’s very important that you don’t battle any other trainers. After defeating the youngster fly to Lavender Town.
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Head west to Route 8 and when you’re at the boundary of Lavender Town and Route 8 you will enter into a wild encounter with a level 7 Mew.
The first time I pulled it off I couldn’t believe it. I didn’t think it would work but it clearly did. Also, it’s not specific to Mew either. The Pokemon that appears is dependent on what Pokemon you defeated last so I think you can also get Gengar and Alakazam with this method.