I've always liked Notational Velocity for OS X. It's minimal but functional. The latest version has some impressive todo features as described here: http://scrodlog.notational.net/notational-velocity-204

I either use a rich text file (that supports strikethrough) or TaskPaper - which is basically a text editor - depending on the context. The added complexity of most existing to-do list software doesn't pay off, for me.

I'm also a big fan of swift to-do list. Really well made and simple to use.

The lite version is a free-to-use download.

http://teuxdeux.com/ is quite handy for simple short term tasks

Emacs + org-mode