Convict Tang
Have one that's been in my tank for about 10 years, has survived everything.
Came in to the LFS almost frozen, survived disease that killed every thing else in my tank except a eel. Has survived heater malfunction (95 degrees), the next day 65. One tough fish.... You really need to Qt, treat and feed the heck out of all new fish. Have had two others over years, one died after a few years when a tank crashed (pH problems) and the other crashed into the glass at full speed one too many times. (close head injury).