It is probably no wonder that the same company that makes the best slap bass guitars also makes the best strings for slapping... I'm talking about Ernie Ball. In fact, before they even owned Music Man guitars, Ernie Ball was a powerhouse known for their awesome strings. Please, don't take my word for it... just ask any beginner, pro or rock-star who makes the best strings. Most of 'em will say Ernie Ball.
Well, that depends on the style of music you play. Generally, the slap bass guitar style benefits from bright, light-gauge roundwound strings made of stainless steel, nickel or an alloy. Also, you'll need to change your strings more often for this style of playing as the crisp articulation of new strings really makes a difference in your slap tone. Generally speaking, the lighter the string, the easier it is to slap. It is not uncommon for pro slappers to change their strings once-a-week... as you can see, slapping is not a cheap technique!