Itachi's genjutsu and/or Tsukoyomi works on one person at each time. But Frog song's genjuts can work on as many people as within its range and it's so powerful. Can it be the reason to say that the frog one is more powerful/effective than the former? What do you think? which one is more effective/powerful?
Depends on what criteria you are judging on. Tsuku's effects take place instantly. Once you are in the genjtutsu, it takes less than a second for it to cause its damage. Their is very little prep time and can be placed just by seeing Itachi's eye. It takes a good amount of chakra but should be able to be used consecutively, if the user is healthy.
Frog song paralyzes the victim(s) in real time. The prep time is very long and can be placed just by hearing it. After it is used, the toads cannot use it again for some time due to their throats recovering.
So it depends on whether you think affecting multiple people with a long prep is more useful or one person at time, heavy chakra cost, and no prep.