Haven't yet found something that ticks all of the boxes..
* Easy task entry with natural language Do thing with Dick and Jane at Location tonight at 6:30 pm and every week on thursday (Sandy was the undisputed queen of this)
* Truly cross platform (Needs a web interface, and an identical native interface on Win/Mac/Lin, and a mobile interface which isn't tied to the web)
* A CLI would be nice but isn't super necessary (ala Taskwarrior)
A couple of my favorite systems get one or two things right really, really welll but then fall flat elsewhere. I'd use Omnitask everywhere except it's completely mac specific, and therefore useless for my needs. Seriously. I would give almost anything to have Omnitask on Windows. Wunderlist isn't bad at all, but isn't terribly stable on the native client, have to enter everything the old fashioned way though. Taskwarrior is my go-to in a *NIX environment but, again, useless anywhere else.
I'm currently using Todoist, which has a decent web interface and mobile client, with semi-decent NLP task entry, and it's doing okay for me now.
I'm very, very seriously open to suggestions here. Asana looks absolutely beautiful, but it's another Mac-Web specific product, so wouldn't work for me.