Fire Red has the most research and tool support. From an objective standpoint, it is the best Rom to hack. However, most things that can be done in Fire Red can also be replicated in Emerald with a little extra work. Both roms have equal potential and capability, but if you want something with more ease of access, Fire Red is your best entry point.
However, if we want to go outside the GBA era, the Retro Games have been disassembled and, if you're willing to learn how to work with the disassembly, Gen 1 and 2 have a lot of amazing potential and you can create some fantastic things with them.