The oldschool goto for ICs has always been FD. I think we tend to get lazy with that c/p a lot, and it's really not the best in several of the high tier mus, but it does force people to have to deal with projectiles, which a lot of characters have trouble dealing with.
It also doesn't have a top platform, which can be nice against spacies (and sometimes falcon, but I really dislike giving those characters ground room as well).
It's really really hard for sheik to handle ICs there, which is nice.
Dizz and a few other climbers are successful on FD against peach, but I think it's really debatable whether or not that stage is actually the best for them, although it does work for the way dizz plays the mu. I think it's horrendous on that stage.
Stadium is like FD but with low side platforms that work for ICs super well. The low ceiling also helps a bit in a couple matchups, but stuff like that is a little arbitrary most of the time.
The transformations are generally garbage for ICs unless they have a reasonable lead. They aren't great at getting around most of them, and are often left in bad positions after the transformations disappear.
But overall the stage is pretty good for ICs.
Most ICs like the small stages a lot too (FoD and Yoshi's). They give them the ability to control space very well and the platforms usually work well for them. These might be their best stages in most matchups with the top 8.
I don't think battlefield or dreamland are horrible for ICs outside of dealing with platform campy players, but I think they are ICs worst of the legal stages most of the time. Some players are very successful on those stages though. I usually ban them against spacies, falcon (or stadium), and sheik.