Real chance - monero. Full anonymous transfer. I think it can second btc.. But btc will be as gold now in fiat currency
What about DeepOnion? Apparently it is slightly better than Monero in some ways. It is new though and most people haven't heard of it, but it is growing every day!
DeepOnion has great potential but BTC will still be a leader anyway. Several times the BTC was about to be defeated but it did not happen.