any kind, as long as it won't fit in the goldfish's mouth. mystery, apple, ramshorn, or nerite.
then again, what size is your tank? goldfish need 20 gallons, minimum just for one. 30g is better. if your tank is under 20g's, i would suggest several snails and a group of white clouds, a coldwater fish which stays on the small side. they are very active and fun to watch.