For the last 10+ years I am using portable WinSnap (it is Windows only) - just 3 Mb, does not need .NET, can conveniently snapshop the current window, all windows of the current application, whole desktop or just a selected area. Has some built-in filters (mirror, border, watermark, negative, grayscale, blur) but most importantly - allows moving around or deleting the annotations individually after their creation (unlike Flameshot). Can not recommend any other screenshoting software.