Tak na szybko:
List<String> list = Arrays.asList("napis1", "napis1", "napis2", "napis3", "napis3", "napis3");
Map<String, Integer> map = new HashMap<>();
for (String key : list) {
map.merge(key, 1, (x, y) -> x + y);
}
String minKey = "";
Integer minValue = Integer.MAX_VALUE;
for (String key : map.keySet()) {
if (map.get(key) < minValue) {
minKey = key;
minValue = map.get(key);
}
}
System.out.println(minKey);