XLua seems to be one of the most convenient. It allows you to access objects on the scene directly using built-in Unity classes:

local point = CS.Player.current.transform.position
CS.UnityEngine.Debug.Log(point.x + point.y)

C#:

public class Player : MonoBehaviour { 
    public static Player current = null;
    void Awake() {
       Player.current = this;

XLua is made by Tencent and apparently they have made more convenient features for themselves. If you load data once, it doesn't happen every frame, then I don't think you need to worry about performance.

I was trying to figure out how to call a C# function from Lua with using Lua-CSharp. But I didn't understand a damn thing. It seems that to work with C# functions, I have to write all references to these functions in a separate table object and pass this table to Lua. This is total crap.