Flood can cause more instant damages. Drought can be avoid by alteration, migration etc.

There is no one answer to this as it depends on the ecosystem and biome. For example, in a flood adapted community, removing the flood would be the ecological disturbance, and in a drought adapted community, increased precipitation would be the disturbance. To answer your question, “it depends.”

@Gazala is saying right heavy flood can not controlled, and drought can be prevent by some early preparation and migration.

floods and droughts are natural hazards. To make them become disasters depends on how well societies are prepared. It is not possible to say, which is more damaging, as such disasters are events of society, and societies differ and are differently prepared to deal with such hazards.

Indeed, there is no one answer. The impact of any of these (and other) incidents is multifactorial and depends on state of preparation, localisation, duration, frequency,speed of onset, geografics, population density.................

Some disasters are controllable if the societies are prepared and use proactive approach.

Lead time in case of floods is very little and thus these are hardly controlled.

Droughts depend upon many factors like land type, micro-climatology of that place, development level of that society as well as financial status of the country. If these factors allow, droughts can be controlled using modern technologies and/or migrating affected people to better places.

Well, by definition of disaster, drought is more devastating than flood. Flood is visible and can be predicted as well. But droughts are very transient and not so obvious to realize. By the time a drought is realized, it is impossible to avoid. Not everything can migrate from drought as it occurs on larger geographical space.

It has lasting impact not only on flora and fauna, but also on groundwater, surface water availability as well.

Many human civilizations died because of long lasting droughts whereas floods were always in human control and have limited impact area.

If you look at the data from Munich Re, it is floods.

--> https://www.munichre.com/topics-online/en/climate-change-and-natural-disasters/natural-disasters.html

In fact, droughts have longer-term consequences than floods whose consequences are direct and on the spot. However, floods may kill more people, animals and plants within a shorter time than droughts. Even the devastation of roads, buildings, fields,...etc. is instantaneous and huge by floods.

The lack of precipitation as well as heavy downpours are causing disasters ranging from extreme droughts to unprecedented floods. Climate change, environmental degradation, population growth, urbanization and augmenting poverty cause human society to be more vulnerable to flood and drought disasters.

Water is a very basic necessity for the survival of life on earth. Imagine life with insufficient amount of water, it will be impossible to do the daily activities of cleaning, cooking, drinking etc. Life will turn out to be a miserable chaos. Water cycle has helped in maintaining the quantity of water on the surface of earth. About 50 liters of water is needed per day per person in order to sustain a healthy life. There are many areas where people do not receive this basic quantity of water. Areas that do not receive adequate amount of rainfall and have dry soil suffer from droughts. Whereas areas which receive heavy rainfall and have marshy soil generally get flooded. Floods and droughts are contrary concepts.

The amount of rainfall received by an area varies from one place to another depending on the location of the place. In some places it rains almost throughout the year whereas in other places it might rain for only few days. India records most of its rainfall in the monsoon season. Rains are a big relief after the hot and sunny days of summers. The growth of crops is also dependent on the arrival of monsoons. However excess rainfall is disadvantageous in many ways. Heavy rains lead to rise in the water level of rivers, seas and oceans. Water gets accumulated in the coastal areas which results in floods. These floods cause extensive damage to crops, domestic animals, property and human life. During floods, many animals get carried away by the force of water and eventually die.

I'm not sure there is a definitive answer to your question - what is your measure of devastation? Loss of lives, or biodiversity, or infrastructure, for example?

On the potential loss of ecosystem services, I can recommend to you a paper by Costanza et al. (2014) which I have used myself - Article Changes in the global value of ecosystem services

You wouldn't go far wrong as to consult Smith's book 'Environmental Hazards' which might help to frame your thinking a little.