What else you planning on keeping, or are you gonna decide that one based onwhat would go with the rare one?
IMHO
This may sound like smart *ss answer but, the rarist thing I ever see is a fish that had room to grow to show size. Most people get a 120 and cram it with fish and they never get an impressive size. If you only get 3 babies that have a large adult size, it may look silly now, but in a year or so it will blow everyone away. Did you ever see a 12" Sailfin Tang? How about a school of 5" Hawaiin Snowflake Damsels, spectacular.
Heres what I would do if I wanted a 120 that looked like no other. First I'd get a Threadfin butterfly. 2 months later a Sailfin or Chevron Tang a little smaller than him. 2 months later I'd get 3 Hawaiin Snowflake Damsels. Then in about 1 month I'd get a juvi net caught Red Sea Pomacanthus angel, a Queen or an Imperator are my favorites. Not a lot of fish but they will get to a size and beauty that is truly rare in the hobby nowadays.