[Popup("String1", "String2", ...] Creates an Enum-style dropdown list of Strings. Attribute in \Attributes, Drawer in \Editor.

PopupAttribute.cs

using UnityEngine;
using UnityEditor;
using System.Collections.Generic;

public class PopupAttribute : PropertyAttribute {

  public List<string> optionsList;

	public PopupAttribute(params string[] args) {
		this.optionsList = new List<string>(args);
	}

}

PopupDrawer.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(PopupAttribute))]
public class PopupDrawer : PropertyDrawer {

  public override void OnGUI (Rect position, SerializedProperty prop, GUIContent label) {
		var attrib = attribute as PopupAttribute;

		int index = attrib.optionsList.IndexOf(prop.stringValue);
		if (index < 0) index = 0;

		int newIndex = EditorGUI.Popup(position, label.text, index, attrib.optionsList.ToArray());

		if (newIndex != index) prop.stringValue = attrib.optionsList[newIndex];
	}
}