[flavor8]

Since nobody has mentioned it, ToodleDo is a good tool with a slightly dated UI and a terrible name. It has mobile apps, 2FA, projects / subtasks / scheduling / optional alarms, and a variety of other related features. There's a free restricted version, also a paid version, which is reassuring.

[dwightgunning]

I'm a long time user and have paid the ~$20/year, subscription for >5yrs. It's been rock solid the whole time.

I find it's got the right out of GTD without being too bloated and/or cumbersome to use.