https://www.myfxbook.com/forex-market/correlation These guys publish a table periodically. Major currencies are strongly correlated, normally. Exotic pairs might offer some breaks in correlation more often.
It changes over time. So the question is difficult to answer as you need kind of system or strategy here. Different lookback periods means different highest correlation. It is not easy to say, better you make your own thoughts, as it is already kind part of your strategy. But for overall there are some candidates for what are you looking for, like EURUSD to USDCHF (inverse relationship) and EURUSD to GBPUSD in second place (for me). Also AUDUSD and NZDUSD are stronger correlated (usually). Last for correlations at all I suggest the following book from Brent Donnelly (25 years experience as trader and chief HSBC currency trader in NYC) as good read here. This one is also a nice read for currency correlations and free pdf on that link from an experienced currency trader a major investment bank for many years. https://traders4traders.com/wp-content/uploads/2018/02/FX-Insider.pdf
It is always said that their is a negative relationship between USD/CHF and EUR/USD and well, it seems that it seems real but do not consider it as a trading tip or a trading system