Sorry for the late response.
The different titles seem to suggest different version of the song.
- 妄想 / Mōsō / Delusion Forest is the version on MEKAKUCITY ACTORS. It doesn't have a video.
- 想像 / Sōzō / Imagination Forest is the version released onto Nico. The audio is slightly different from Delusion Forest.
- 空想 / Kūsō / Fantasy Forest is the version released with MEKAKUCITY DAYS. The audio is the same as Imagination Forest, but it has a slightly different video. The only major difference is the ending (you can view a video I made with them side by side here). It's worth noting that the anime episode, as well as the cover present on MEKAKUCITY M's, is also referred to by this name.