Really cool and I always love these types of apps. My personal favorite is TickTick right now. It is basically cross platform on all OS's, save for Linux (but you can use a browser extension too so it works). The downside it's account based so in order to sync you must have an account.
Also there are some cool features that are only accessible if you become a subscriber. On Android alone, the widgets they've made are spectacular. You can hit a checkbox and it disappears, color toward priority, and organize by date due. Mac has a similar popup as well which is very nice. Windows you need to open the app but it can also be in the overflow panel.
It is the best cross platform TODO software I've found.