What is better for a democracy - a Prime Minister who can do whatever he wishes or a Prime Minister who needs to consult other leaders and parties?
Both situations mentioned in the question are the extreme cases. One in which the Prime Minister does whatever he wishes and the second case in which the Prime Minister consults other leaders and parties. If a Prime Minister is free to do as he wishes, there is a possibility that he might develop authoritarian or dictatorial tendencies.
On the other hand, if all the time he has to consult other leaders and parties, it would affect his working. Therefore, a Prime Minister should have freedom of taking decisions, but at the same time, he should also consult with other leaders and parties on important issues