I'm probably gotta get some somewhat deserved heat for that one but for me it's Temple of Elemental Evil. On one hand it's an adaptation of a classic module, which brought it to the wider audience who's not really into retro modules. Also it's probably the most faithful adaptation of DnD and it should become a template of how future RPGs should play like. Giving the party entirely different justification for
On the other hand the plot is rather bland, most of the combat encounters are a slog, NPCs aren't memorable, Hommlett is extremely boring, bosses are disappointing, the quests aren't too interesting, the entire setting is rather banal. Great game systems are brought down by rather underwhelming content. Too bad it didn't have an option to create your own modules like Neverwinter Nights did. It's not a bad game but any means. I just don't feel that it deserve to be presented as one of THE greatest RPGs ever.
I put it over games like Original Sin, Witcher 3, Disco Elysium etc. since these have their own dedicated haters which makes them less overrated in comparison. I don't consider them better though.