I find the BitcoinWisdom predictor to be fairly reliable.  It doesn't swing too much.  I don't know what algorithm they use, but it's resistant to at least some of the flaws that I've seen with others.  I think it must use some kind of upward bias where it more heavily weights the most recently seen high-point for hashrate.  This works on an intuitive level because while new mining hardware might be turned on for testing then taken offline while it's shipped out to customers, after a week or two it will tend to turn on and stay on permanently.  So if the network hits a hashrate peak and then drops down, it's a safe bet that it will soon hit that level again.

https://bitcoinwisdom.com/bitcoin/difficulty