Five countries that have been mostly affected by tsunamis are:

1. Indonesia
2. Japan
3. Sri Lanka
4. Thailand
5. India

Indonesia:
Indonesia has been hit by several devastating tsunamis in the past few decades, including the 2004 Indian Ocean tsunami, which caused widespread destruction and claimed over 230,000 lives. The country is located on the Pacific Ring of Fire, which makes it vulnerable to earthquakes and volcanic eruptions that can trigger tsunamis.

Japan:
Japan is another country that is prone to tsunamis due to its location on the Pacific Ring of Fire. The country has experienced some of the worst tsunamis in history, including the 2011 Tohoku earthquake and tsunami, which killed over 15,000 people and caused widespread damage to the country's infrastructure.

Sri Lanka:
Sri Lanka was one of the countries that was hit hardest by the 2004 Indian Ocean tsunami, which killed over 30,000 people and caused extensive damage to the country's coastline. The country has since implemented a number of measures to improve its tsunami warning system and to better prepare for future disasters.

Thailand:
Thailand was also hit hard by the 2004 Indian Ocean tsunami, which killed over 5,000 people and caused extensive damage to the country's tourism industry. The country has since implemented a number of measures to improve its disaster preparedness, including the construction of tsunami warning systems and the development of evacuation plans.

India:
India has been hit by several tsunamis in the past, including the 2004 Indian Ocean tsunami, which killed over 10,000 people and caused extensive damage to the country's coastal areas. The country has since taken steps to improve its disaster preparedness, including the establishment of a National Centre for Ocean Information Services (NCOIS) to monitor ocean conditions and provide early warnings of potential disasters.