If the 120 is a 4' tank, any of the ctenochaetus.sp are good - kole tang, bristletooth tang, Chevron tang, etc. A few of the zebrasoma.sp would be OK - yellow tang, purple tang, black tang.
Stay away from any tang that gets bigger than 12". Some of the ancanthurus.sp stay under 12", but they are either very aggressive or very active swimmers...or both. A 4' tank is not the ideal size tank for a tang.