PrimitiveType.Sphere  

// Creates a sphere primitive
function Start () {
    var sphere : GameObject = GameObject.CreatePrimitive(PrimitiveType.Sphere);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        GameObject sphere = GameObject.CreatePrimitive(PrimitiveType.Sphere);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        sphere as GameObject = GameObject.CreatePrimitive(PrimitiveType.Sphere)