SortingLayerを設定する。ShaderにTransparentとかParticle使っている場合はコレで描画順が制御出来る

SortingLayer.cs

/// Set Sorting Layer
/// Copyright (c) 2014 Tatsuhiko Yamamura
/// Released under the MIT license
// / http://opensource.org/licenses/mit-license.php

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

#if UNITY_EDITOR
using UnityEditor;
#endif

[ExecuteInEditMode, RequireComponent(typeof(Renderer))]
public class SortingLayer : MonoBehaviour
{
	[SerializeField, SortingLayerAttribute]
	private string
		layerName = "Default";
	[SerializeField]
	private int
		orderInLayer = 0;
	private Renderer _renderer;
	
	void Awake ()
	{
		LayerName = layerName;
		OrderInLayer = orderInLayer;
	}
	
	void OnValidate ()
	{
		LayerName = layerName;
		OrderInLayer = orderInLayer;
	}
	
	public string LayerName {
		get {
			return layerName;
		}
		set {
			layerName = value;
			foreach( var renderer in GetComponents<Renderer>() )
			{
				renderer.sortingLayerName = layerName;
			}
		}
	}
	
	public int OrderInLayer {
		get {
			return orderInLayer;
		}
		set {
			orderInLayer = value;
			foreach( var renderer in GetComponents<Renderer>() )
			{
				renderer.sortingOrder = orderInLayer;
			}
		}
	}
}

class SortingLayerAttribute : PropertyAttribute
{
}

#if UNITY_EDITOR
[CustomPropertyDrawer(typeof(SortingLayerAttribute))]
public class SortingLayerDrawerAttribute : PropertyDrawer
{
	private static SerializedProperty sortinglayer = null;
	public static SerializedProperty SortingLayer{
		get{
			if( sortinglayer == null )
			{
				var tagManager = new SerializedObject(AssetDatabase.LoadAllAssetsAtPath("ProjectSettings/TagManager.asset")[0]);
				sortinglayer = tagManager.FindProperty("m_SortingLayers");
			}
			return sortinglayer;
		}
	}
	
	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
	{
		var list = AllSortingLayer;
		var selectedIndex = list.FindIndex(item => item.Equals(property.stringValue));
		if( selectedIndex == -1)
			selectedIndex = list.FindIndex(item => item.Equals("Default"));
		
		selectedIndex = EditorGUI.Popup(position, label.text, selectedIndex, list.ToArray());
		
		property.stringValue = list[selectedIndex];
	}
	
	private List<string> AllSortingLayer
	{
		get{
			var layerNameList = new List<string>();
			for(int i=0; i<SortingLayer.arraySize; i++)
			{
				var tag = SortingLayer.GetArrayElementAtIndex(i);
				layerNameList.Add( tag.displayName );
			}
			return layerNameList;
		}
	}
}
#endif 

TransparentやAlphaBlendのシェーダーを使ったマテリアルならば、こんなかんじで描画順を制御出来る。
ParticleRendererとSpriteRendererの描画順を制御する際に利用する。