Did a quick check,

Apparently gsc max atmos at mytown (hall 3) and mbo big screen at Starling have almost the same screen size (20 metres x 11 metres).
Note that the screen size means that to enjoy the largest screen possible, the movie needs to be in 1:1.85 aspect ratio, which is a problem because most modern blockbusters are 1:2.35 in aspect ratio (ghost in the shell is the only recent blockbuster movie I can think of that is in 1:1.85). This means most of the movie you watch in these big halls, the top and bottom of the screen is not fully utilize. The only other upcoming blockbuster I think that is projected in 1:1.85 is justice league (based on the trailer).
Both MBO and GET use Dolby Atmos, and based on memory alone, the sound system is Imax hall is way better. Picture quality wise I don't think there's much difference (not sure if there's any discernible picture quality for movie that are shot using Imax cameras).

Lastly, I Have no idea what is the screen size of TGV velocity Imax size (it boasts that it has the largest Imax hall in malaysia, not cinema hall so I'm pretty sure it's at least relatively smaller than GSC mad almos and mbo big screen). But pretty sure for pure moviegoing quality, Imax is still better (but for us layman viewer, we might not notice any difference). The offset is of course the pricing. IMAX is generally priced around rm 5 more expensive than their GSC and MBO counterparts.