[quote name='argyle']It's hard to narrow it down to just one, but if I had to it would probably be:
Rhapsody for the PSOne, mint & complete w/ soundtrack. I snagged this off of Ebay before Disgaea hit & it became sought after.
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A small game store in the middle of nowhere I go to had two sealed copies of rhapsody a little over a year ago (they have lots of old, sometimes rare, sealed ps1 games). I already had a copy so I bought them for 30 each and sold them on ebay for about $120 (great when considering most went for 60 at the time when they showed up), if only I had waited a few more months. Though my pride and joy would be my "donkey kong country competition cart", paid $20 in nintendo power, seen it go for $3-450 on ebay. After that a complete and mint spiderman web of fire (32x) I only recently found out was extremely rare and had value (either bought it on clearance or when it first came out, since I bought the 32x when they were discounting them and spiderman was one of the last games released), then panzer dragoon saga (had it reserved so I got it the day it came out). The original Lufia is my favorite game, but it's easily replaced. If I ever get magical chase or dynastic hero (both for turbo duo) then those would be my picks for second. Buying things and later finding out they have value makes them more important to me, anyone can just pay alot for a rare game, but it takes dumb luck to get them when they were first released. I'd also love battlesphere for jaguar, but that came out at $200 and whenever they rerelease it its gone before I notice.
EDIT: Growlanser box set came out? Damnit, I had that preordered and they never called me. Think it's too late? Also, valkyrie profile, is it good for someone who likes the old style, slow, repetitive turned based rpgs (dragon warrior, lufia, lunar etc.) games but never really enjoyed the newer rpgs (direction rpg's went after post-ff7 style)? The only new rpg's I tend to like now are strategy, like disgaea, and 2d gameboy ones. It's strange, but I hate battle systems that are time based, where you can run around freely etc. I never grew out of the old style. And I also love world maps, the ones where you walk from town to town, dungeon to dungeo.