I don't think you'll find one. The best sandbox is no sandbox. It's always a hindrance when you have to shift focus from the linear-ish story into this incoherent chaos of buttons that don't do anything, the need to match certain time of day and/or weekday and of course the complete lack of information as to what can be done, because apparently no dev has ever played a sandbox game before, since they all do the same damn mistakes.
However there are few exceptions that barely are considered as sandbox, which makes them ok in my book:
- Desert Stalker
- You have a clear quest log, almost no need to roam around and markers are clearly labeled when there is an available quest on that location. Not to mention the story is excellent.
- Lust theory
- I don't remember for 100%, but I believe there was a free-roam in terms of you can choose which girl to pursue and you always had some event available. But don't quote me on this one, it's been a few years.
Also as
@zark already mentioned the headmaster is rather fun if you are able to push through the free-roam clicking annoyance and some randomly triggered events