Oh dear, so many choices... I'd go for the Gold Box series, if only because they introduced me to the genre at a time where everyone in my social circle was keen on Doom et al. In particular, Curse of the Azure Bonds and Champions of Krynn - for something more modern, I'd say either Deus Ex or the Fallout series.
[sub]Although Deus Ex isn't quite-exactly-specifically an RPG, it just does so many things *right*, y'know?[/sub]
[sub]In lieu of avoiding misunderstandings, Doom was pure awesomesauce at the time, mkay?[/sub]