I went with gameplay and story.
I just got three words for ya: Final. Fantasy. Seven.
By today's standards, the audio is laughable (no speaking voices!?), the graphics are childish (it's all chibi!), the music is *so* MIDI-synth, and it's hardly the first RPG ever released... but the story, the game-mechanics, and the re-playability make it the greatest title coded to date (IMO).
With the current break-up between Square-Enix and Sony... hell, if S-E re-released FF7 with FFX-style graphics and voices, I'd buy an Xbox-360 or a Nintendo Revolution *just* for that game. No changes to plot, story, or mechanics - just modernized graphics and audio. I'd buy a whole console just for that.
Metroid was/is the greatest series ever made, but FF7 is the best standalone title ever, and that's just on the strength of the plot and the game-mechanics.