Todoist! I use it every day and it's absolutely superb. It's cross platform, allows you to manage different projects for your personal and professional to-do's, and with premium you can get really advanced with labels. It's so easy to use and is an absolute life saver. It also works offline incase you remember something you need to add when you are on the go without an active internet connection. The UI is stunning on desktop web, and on the Android app.