alot people are saying bitcoin but the op have just $500 to invest in altcoin. first we need to know how long you are planning to invest your $500. then I will suggest you do your own research and probably invest your money on coin with price less than 1$. XRP is good if you are thinking of a long term investment. IOTA, NEM are some other few coin which are cheap and can offer you good profit if you invest. for short term investment you may consider waves and probably NEO