Most efficient way to find the closest collider to a player?

Awesome, thanks guys. Here is the full code for what I ended up doing. I haven't seen any complete code to mantle onto an arbitrary object semi-reliably, so hopefully this helps someone.

 public void Mantle(float maxHeight, float maxDistance) {
 
     Vector3 playerPos = controller.transform.position;
 
     //Check to see if an object is selected
     if (selectedObject)
     {
         //Find the collider closest to the character
         Collider[] colliders = selectedObject.GetComponentsInChildren<Collider> ();
         Collider closestCollider = colliders[0];
         Vector3 closestPointB = closestCollider.ClosestPointOnBounds (playerPos);
         float distanceB = Vector3.Distance (closestPointB, playerPos);
 
         foreach (Collider collider in colliders)
         {
             Vector3 closestPointA = collider.ClosestPointOnBounds (playerPos);
             float distanceA = Vector3.Distance (closestPointA, playerPos);
 
             if (distanceA < distanceB)
             {
                 closestCollider = collider;
                 distanceB = distanceA;
             }
         }
 
         Vector3 closestPoint = closestCollider.ClosestPointOnBounds (playerPos);
 
         //Find the top of closest collider
         float colliderHeight = closestCollider.bounds.max.y;
         
         //Debug.Log (closestCollider + " @ " + colliderHeight);
 
         // Check range of object;
         if (new Vector2 (closestPoint.x - playerPos.x, closestPoint.z - playerPos.z).magnitude > maxDistance || // Too far away!
             colliderHeight - playerPos.y > maxHeight || //Too high!
             colliderHeight - playerPos.y < -1) //Can't mantle down!
             return;
 
         /*
         Debug.DrawRay(new Vector3 (closestPoint.x + 0.0F, colliderHeight, closestPoint.z + 0.0F), new Vector3 (0.0F, 2.5F, 0.0F), Color.red, 2.0F);
         Debug.DrawRay(new Vector3 (closestPoint.x + 0.5F, colliderHeight, closestPoint.z + 0.5F), new Vector3 (0.0F, 2.5F, 0.0F), Color.green, 2.0F);
         Debug.DrawRay(new Vector3 (closestPoint.x - 0.5F, colliderHeight, closestPoint.z - 0.5F), new Vector3 (0.0F, 2.5F, 0.0F), Color.blue, 2.0F);
         Debug.DrawRay(new Vector3 (closestPoint.x - 0.5F, colliderHeight, closestPoint.z + 0.5F), new Vector3 (0.0F, 2.5F, 0.0F), Color.yellow, 2.0F);
         Debug.DrawRay(new Vector3 (closestPoint.x + 0.5F, colliderHeight, closestPoint.z - 0.5F), new Vector3 (0.0F, 2.5F, 0.0F), Color.magenta, 2.0F);
         Debug.DrawRay(playerPos, new Vector3 (0.0F, 4.0F, 0.0F), Color.white, 2.0F);
         */
 
         // Check if it is reasonable to climb on the area selected.  This doesn't guarantee the player will fit, just that there is enough room to try.
         if (Physics.Raycast(new Vector3 (closestPoint.x + 0.0F, colliderHeight - 0.01F, closestPoint.z + 0.0F), Vector3.up, 2.5F) ||
             Physics.Raycast(new Vector3 (closestPoint.x + 0.5F, colliderHeight - 0.01F, closestPoint.z + 0.5F), Vector3.up, 2.5F) ||
             Physics.Raycast(new Vector3 (closestPoint.x - 0.5F, colliderHeight - 0.01F, closestPoint.z - 0.5F), Vector3.up, 2.5F) ||
             Physics.Raycast(new Vector3 (closestPoint.x - 0.0F, colliderHeight - 0.01F, closestPoint.z + 0.0F), Vector3.up, 2.5F) ||
             Physics.Raycast(new Vector3 (closestPoint.x + 0.0F, colliderHeight - 0.01F, closestPoint.z - 0.0F), Vector3.up, 2.5F) ||
             Physics.Raycast(playerPos, Vector3.up, 4F))
             return; // No room to fit!
 
         //Debug.Log ("Can mantle this");
 
         Vector3 mantlePoint = new Vector3 (closestPoint.x, colliderHeight + 2.33F, closestPoint.z);
 
         //Make character face object being mantled
         controller.transform.LookAt(new Vector3 (closestPoint.x, playerPos.y, closestPoint.z));
 
         StartCoroutine (moveCharacter (controller, mantlePoint, mantleMaxTime));
         CrouchCharacter();  //This allows character to climb up to tight areas; e.g., an air vent or plenum
     }
 }
 
 public IEnumerator moveCharacter (CharacterController character, Vector3 newPos, float maxTime) {
     //Debug.Log ("Trying to move character to " + newPos);
 
     //Make sure the player doesn't have any movement going into the mantle or things break
     //I also disable movement input while mantling
     moveDirection = Vector3.zero;
 
     //We don't want gravity to interfere with our climbing
     float oldGravity = gravity;
     gravity = 0.0F;
 
     //Start moving player up only.
     float t = 0.0F;
     while (t < maxTime && character.transform.position.y < newPos.y){
         Vector3 dir = (newPos - character.transform.position).normalized * mantleSpeed;
         //Debug.Log (dir);
         character.Move (new Vector3 (0, dir.y, 0));
 
         //Once player is in stair-height range of y, we can start moving them forward
         if (character.transform.position.y > newPos.y - 1)
             character.Move (dir);
 
         t += Time.deltaTime;
         yield return new WaitForFixedUpdate();
     }
 //Nudge the player just a little bit to make sure they stay on.
 
         for (int i = 0; i <= mantleNudgeFrames; i++) {
             character.Move (new Vector3 (nudgeDirection.x, 0, nudgeDirection.y).normalized * mantleSpeed);
             yield return new WaitForFixedUpdate();
         }
 
     gravity = oldGravity;
 
 //Attempt to stand back up
 if (canStand)
     UncrouchCharacter ();
 
 yield break;
 }