Immortal Emperor is Nine Worlds top cultivation, Grand Emperor is the Tenth World equivalent. However, it is to be noted that one Heaven Will from the Nine Worlds is worth two in the Tenth World. Immortal Emperors who ascend don't mix well the same as natural born inhabitants and can only obtain about two Heaven's Wills from the Tenth World because of this. (I don't think this is the max, but Immortal Emperors have to ascend quickly to avoid being killed by a Heavenly Execution and no one has seen an Immortal Emperor with 12 wills.) Grand Emperors, however can get 12 max, with three attempts to gain Heaven's Wills. They're considered a genius based on how many they can catch at one time (Four being the greatest). As I've said, three attempts to gain Heaven's Wills, although I don't remember the reason for this. Immortal Monarchs are the exact same thing as Grand Emperors, but they are from the hundred races. This is why Gu Guo is a Immortal Monarch, a human is considered one of the hundred races. Emperors from the Three races, Heaven Race, Devil Race, Divine Race are called Grand Emperors, so if you see the words 'Grand Emperor' you can immediately tell the person being referred to is from one of the Three Races. If not, you can tell they are from one of the hundred races.

Hope this answers your question. And you're welcome. :)