Custom inspector with a dropdown

TriggerEditor2.cs

using UnityEngine;
using UnityEditor;

[CustomEditor (typeof(Trigger))]
[CanEditMultipleObjects()]
public class TriggerEditor : Editor
{
	private SerializedObject obj;
	private SerializedProperty radius;
	private SerializedProperty priority;
	string[] options = {"high", "medium", "low"};
	int index = 0;

	public void OnEnable ()
	{
		obj = new SerializedObject (target);
		radius = obj.FindProperty ("radius");
		priority = obj.FindProperty ("priority");
		if (priority == null)
			index = 0;
		else	
			index = priority.intValue;
	}

	public override void OnInspectorGUI ()
	{
		obj.Update ();
		GUIStyle style = new GUIStyle ();
		style.font = EditorStyles.boldFont;
		EditorGUILayout.LabelField ("Trigger", style, null);
		EditorGUILayout.PropertyField (radius);

		if (radius.floatValue < 0) {
			radius.floatValue = 0;
		}
        
                // Make a horizontal layout for priority
		EditorGUILayout.BeginHorizontal ();
		EditorGUILayout.LabelField ("Priority");
		index = EditorGUILayout.Popup (index, options);
		EditorGUILayout.EndHorizontal ();
		priority.intValue = index;
		
		obj.ApplyModifiedProperties (); 
	}
    
	public void OnSceneGUI ()
	{
            // Implement what you want to see in scene view here
	}
}