I mainly go with the selection mentioned on
this wiiflowiki page. It mentions the version, base IOS and slot. The download link has a package to d2x version 8 final, which is most likely better than the mentioned beta r41 package.
As mentioned, the IOS'es with bases 56 and 57 will be enough for roughly 99,9% of the games (I remember one that only worked on 58, but that was an unique one...and some d2x versions ago). Note that the "added functionality" doesn't hinder anything if it's installed in other slots. So even though I probably never use those in slots 245 to 248, it doesn't hurt to have them.
@[member='kirbymaster101']: you have no idea what you're talking about, right? "249" is just the name of the slot. I can install my mother to slot 249 and call her cIOS249...it doesn't make her a recommended cIOS to load anything. Even if she was based on IOS56.