That's extremely vague and subjective. Most fan games & rom hacks can be considered fair or at least have one fair difficulty setting.
If you want fair and balanced difficulty, any Pokémon game that's either not too challenging, or that is challenging but gives you the tools to overcome that difficulty at a good pace should probably work. Basically, just avoid Kaizo games or games with troll difficulty in which opponents can pull illegitimate BS tricks and advantages out of nowhere, get access to way broken mons and moves early on, etc.