Here's a few ideas:
-(I realize this has already been suggested, but I'll suggest it again)
Higurashi. Best played as a horror game in the dark with headphones on full. Great story line.
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Symphonic Rain Haven't read this yet, but is highly rated so I thought I'd suggest it anyway.
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Planetarian a rather short key visual novel - and this is actually available legitimately on IoS in English so support key!.
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Sharin no Kuni, Himawari no Shoujo Despite the really long title, a pretty awesome visual novel. Compelling story with nice characters.
For other ideas, check out vndb and sort by ranking:
http://vndb.org/v/all?q=;fil=tagspoil-2;o=d;s=rating
Anything on the first or second pages will probably be pretty good. Hope this helps!