There's nothing wrong with nulls if you have proper types :)

max.kt

fun max<T : Any>(list: List<T>, weight: (T) -> Int): T? {
    return when (list.size()) {
        0 -> null
        1 -> list.first()
        else -> list.reduce({ a, b -> if (weight(a) > weight(b)) a else b })
    }
}

fun main(args: Array<String>) {
    println( max(listOf("a", "bcd", "ef"), { it.length() }) )
    println( max(listOf("a"), { it.length() }) )
    println( max(emptyList<String>(), { it.length() }) )
    println( max(emptyList<String>(), { it.length() }) ?: "default" )
}

Play with it at try.kotlinlang.org.