Pokemon Crystal is the updated "third" version of Pokemon Gold and Silver. This revision introduces a few hallmarks that continued onwards through the series: Pokemon sprite animations, the Battle Tower, a female protagonist choice, and even legendary-focused storylines through a little side story surrounding Suicune. Colors and the like were also updated for a bit more quality, as Crystal was also on a GBC only cartridge.

This is pretty much Gold or Silver but even better, you can't really go wrong with it.

Twitch Plays Pokemon Anniversary Crystal is an extensive ROM hack created for the stream's second anniversary.

It contains a new storyline combining parts of Pokemon Red and Pokemon Crystal, with some new elements.
The post-game Kanto portion of the original Crystal has been greatly expanded with new quests.
The player is able to catch all 251 Pokemon as well.

The gameplay has also been revamped with elements from later games as well. Physical/Special damage split (with new moves from newer games), Fairy Type, Move Re-Learner, EXP for catching have been included. Original game bugs have been fixed, such as with specialty Poke balls.

Of note is that the game optionally includes an "Advanced AI" hack. However, this will require a special emulator setup to install, as it allows an external script to analyze the player and enemy data to pick smarter moves for the AI. As RHDN does not permit emulator-specific hacks, that variation of the patch will need to be download at the authors' site, http://tppdevs.com/ .
The standard version of the hack, which uses the original AI, can be applied and run in any emulator.

The "standard" patch can be applied to an English version 1.0 Crystal ROM.
It is known as the "USA, Europe" ROM in No-Intro, and as the "USA" ROM in GoodTools.
CRC32: EE6F5188
MD5: 9F2922B235A5EEB78D65594E82EF5DDE
SHA-1: F4CD194BDEE0D04CA4EAC29E09B8E4E9D818C133

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