Either rarity/value wise, or as a memorable piece in your collection:
For me, it would have to be Panzer Dragoon Saga and MUSHA. I got them both under different circumstances. PDS was purchased about at the market going rate a few months ago ($230 complete--now it's a bit higher); and MUSHA, I found at my local Goodwill for .$75 cents, cart only. This goes to show there are still good finds at Goodwill--you just have to be there at the right time/place.
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As far as a memorable piece in my collection, I would say Xenogears, which I got around 2000/2001 for $55 used. Back then, the game was quite rare. I have such fond memories of this game, waking up early (6-7am) and playing it at my parents' house (Kitchen TV, didn't have a TV in my room at the time) until almost noon. Before even getting the game, I would play the demo bundled with Parasite Eve over and over again! I even remember where it ends and I've watched the intro fmv sooooo many times!
I don't replay games too many times (much less RPGs), but Xenogears I think I played 2-3 times since then.