An example of how interaction could be done. Untested code.

InteractionExample.cs

using UnityEngine;


public class InteractionExample : MonoBehaviour
{
	public string interactionKey = "i";
	public float interactionRadius = 2.0f, interactionViewCone = 60.0f;
	
	
	void Update ()
	{
		if (Input.GetKeyDown (interactionKey))
		{
			Interact ();
		}
	}


	void Interact ()
	{
		Collider[] colliders = Physics.OverlapSphere (transform.position, interactionRadius);
			// Might want to apply a layer mask here, to reduce the cost of the query

		if (colliders.Length < 1)
		{
			return;
		}

		float shortestDistance = Mathf.infinite;
		GameObject closestInteractive = null;

		foreach (Collider collider in colliders)
		{
			GameObject root = collider.transform.root;
			Vector3 direction = root.transform.position - transform.position;

			if (root.tag == "Interactive" && // Or whatever means you use for marking objects as interactive
				Vector3.Angle (direction, transform.forward) < interactionViewCone
					// Don't interact with stuff behind us
			)
			{
				if (shortestDistance > direction.sqrMagnitude)
				// Find the closest interactive object
				{
					shortestDistance = direction.sqrMagnitude;
					closestInteractive = root;
				}
			}
		}

		if (closestInteractive != null)
		// Interact if we found a good candidate
		{
			closestInteractive.SendMessage ("OnInteract");
		}
	}
}