What is the best ROM to hack nowadays?

I would like to know your opinion, remember to give a reason why you think that that ROM is the best to such day as today.

Pokemon FireRed is the most hacked Rom in PokeCommunity i saw.
There are many other tools for FireRed that aren't for R/S/E. Also, The graphics went more higher in FireRed than in R/S/E.

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.

EMs got more stuff now I think. But EM's default graphics also looks like it's from a side scroller...so yeah.

FireRed is the easiest to work from since it is already such a base game in its own. It's maps and events are very standard and not too complex so it shouldn't be difficult working from them to create something new. FRLG also has the most support for GBA ROM Hacking tools. Especially tools that handle the metadata and creative assets.