My choice is Way of the Wicked by Fire Mountain Games.
http://rpg.drivethrustuff.com/product/97880/Way-of-the-Wicked-Free-Preview%3A-Prison-Break%21
I'm primarily a 3e/pathfinder player, so don't know about most other edition adventures.
I considered a few Paizo APs as well as several products from Open Design (both/all of which are stellar, rating 3+ through 4 or even 5 stars).
However, in terms of a "new classic" that meets the categories of: innovative, interesting, fun, well written, and exciting I couldn't find a competitor that outdid Way of the Wicked.
(Please note, I'm not affiliated with them in anyway, nor am I a fanboi based on their prior works or reputation, as they have neither prior works nor reputation to my knowledge.
That doesn't mean I won't gush about this AP...I'm lovin' it.)
MAN! It's an evil adventure path done right for the only time I've ever seen it done right in any edition. While railroady at first (to get those evil PCs in line), it quickly becomes more sandboxy with players getting 1. their own dungeon to flesh out and defend against meddlesome good guys, and 2. their own evil organizations.
As a bit of a grammar nazi myself, I'll admit that there are a few (through several) grammatical errors depending on the book, but overall, I am deeply inspired to run this campaign...and not just because it finally does an evil campaign well...because it's the best campaign I've seen in a while that just happens to be evil.
If you haven't checked it out, you should. You'll be doing yourself a favor.
All that said, I'll be quiet now other than asking questions about others' choices and answering what I can about that AP, though Gary McBride is one of two authors and has an account on this site. His account name is @
gdmcbride
So now...please gush away about your own favorites so I can go out and buy em! (If you could also provide links to read more about them/buy them that'd be appreciated as well).
Also, close second choice was a tie between Slumbering Tsar and the new Rappan Athuk (reloaded sorta) Kickstarter. But, while those each get 5 stars, I don't think they get credit for the level of originality that Way of the Wicked does, IMO. It's a tough life comparing 3 five star products, but I guess I'll live.