It will probably depend what you want to do and how good your skills are. Also depends what is out there for them -- Zelda games are fairly popular for hacking and so a lot of documentation has been done for the bigger ones (thinking http://datacrystal.romhacking.net/wiki/The_Legend_of_Zelda
), however something like GBA four swords is relatively unknown.
If you read something like Final Fantasy 3 (Japanese numbering) is not hacked very much because it takes up the whole cartridge and does not have space for extras and want something for Zelda then you might get something. However while lacking space for extras will make it hard to make the next https://www.romhacking.net/hacks/197/
it does not stop you from altering drops, damage, tweaking the locations of a few existing things and otherwise profoundly altering the game.
None of the NES, SNES, GB/GBC or GBA efforts have any weird and wonderful programming styles that change how the game works between levels (like some other games do from time to time). While most probably would not pour heaps of praise on Nintendo's coding skills the Zelda games from what I have seen are mostly "build an engine, run with it" than chains of effectively minigames.
None have any particularly notable compression, encryption, protection... that you will be constantly fighting with if you are playing hacker.
If you are looking for a challenge then you can do that for any of them -- always more documentation of how the games work welcome.