Over the past couple of years the MTA's real-time data has gotten much, much better. Most stations have screens showing when the next train(s) are arriving, including signs outside the station near entrances. (now are they always working, that's a different question).

I live in Brooklyn and my local train (C) is one of the worst for reliability. It comes every 6-15 minutes during commutes, so it's important to know exactly when the next train is coming so you can hurry (or chill, if you have already missed the train!). https://mta.info is the "horse's mouth" and, while it's not a great UX, I much prefer it to going to any third-party repackager's app or site.

Just for finding your way around the city, no need to look further than Google Maps on mobile, which gives clear and reliable subway directions, including scheduled service interruptions. The only asterisk on that is that temporary service interruptions are not always taken into account for finding directions, but mta.info will always set you straight.