It's a solid, resounding "ARRRRRRR!" "YAAARRRRR!" is acceptable as well. G's and H's are strictly frowned upon, unless used in the interjection "ARRRRRGGGHHHH!", which is a slightly less-eloquent way of saying "Blast to the depths of Davey Jones' Locker and back again, sittin' upon a belayin' pin smarts like the devil!"
However, truth be told, the specific spelling of the word is rather useless to fret over. One doesn't attack a merchant vessel and calmly type "ARRRR!" That's a different type of pirating altogether. No, the real importance lies in how you actually pronounce it-- and as long as it's loud and from the depths of your bowels, you're good.