ClosestObject Brute Force

using UnityEngine;

public class Closest : MonoBehaviour
{
    [Tooltip ( "The array of objects to check." )]
    public Transform [ ] PArray;

    public Transform ClosestP { get; private set; } = null;

    [Tooltip("A default of '1' indicates that we check once a second for the closest object.")]
    public float CheckInterval = 1;
    private float timer = 0;

    private void Start ( )
    {
        // Make sure that we calculate the distance once at the very beginning.
        timer = CheckInterval + float.Epsilon;
    }

    void Update()
    {
        timer += Time.deltaTime;
        if ( timer >= CheckInterval )
        {
            timer -= CheckInterval;

            var closestPIndex = ClosestPIndex ( );

            if ( closestPIndex >= 0 )
                ClosestP = PArray [ closestPIndex ];

            // ClosestP is now your closest object.
        }
    }

    private int ClosestPIndex ( )
    {
        var closestIndex = -1;
        var closestDistance = float.MaxValue;
        for ( int i = 0; i < PArray.Length; i++ )
        {
            var sqrMagnitude = ( transform.position - PArray [ i ].position ).sqrMagnitude;
            if ( sqrMagnitude < closestDistance )
            {
                closestDistance = sqrMagnitude;
                closestIndex = i;
            }
        }
        return closestIndex;
    }
}