C# 2D platformer movement code

There are a couple of ways to achieve this but following your current implementation: why not set the isGrounded flag to false when you jump and just let the trigger handle resetting the flag when you land?

Some things you could check: Are all your colliders 2D and set to be triggers?
Have you checked your layer collisions are actually happening?

public class PlayerController : MonoBehaviour 
{

    //Movement
    public float speed;
    public float jump;
    float moveVelocity;

    //Grounded Vars
    bool isGrounded = true;

    void Update () 
    {
        //Jumping
        if (Input.GetKeyDown (KeyCode.Space) || Input.GetKeyDown (KeyCode.UpArrow) || Input.GetKeyDown (KeyCode.Z) || Input.GetKeyDown (KeyCode.W)) 
        {
            if(isGrounded)
            {
                GetComponent<Rigidbody2D> ().velocity = new Vector2 (GetComponent<Rigidbody2D> ().velocity.x, jump);
                isGrounded = false;
            }
        }

        moveVelocity = 0;

        //Left Right Movement
        if (Input.GetKey (KeyCode.LeftArrow) || Input.GetKey (KeyCode.A)) 
        {
            moveVelocity = -speed;
        }
        if (Input.GetKey (KeyCode.RightArrow) || Input.GetKey (KeyCode.D)) 
        {
            moveVelocity = speed;
        }