1st edition Red Cheeks Pikachu would indeed be the first I would assume.
Noteworthy there are also back-imprinted 1st edition error (both in black and white) for the Red Cheeks Pikachu, so a double error if you would count Red Cheeks Pikachu as an error. I personally just deem Red Cheeks Pikachu as a print variation instead of an actual misprint though, since a.f.a.i.k. WotC knowingly printed them with red cheeks since they thought Pikachu = red cheeks, but then changed it to yellow cheeks based on the original Japanese design later on. This has never been confirmed or denied though, so it’s indeed commonly known as an error. 
After that, 1st edition Dragonaire with rainbow error and 1st edition Vulpix with blue butt would come before any Shadowless errors, though. 
Here two sources with lists of known misprints:
- Masters Guide for Pokemon WOTC Corrected Errors & Test cards
- Error cards - Bulbapedia, the community-driven Pokémon encyclopedia
Greetz,
Quuador