Unfortunately, not just one. My two prized PS1 RPG saves:

Legend of Mana - Lv. 99 Kiro (Boy) (and maybe, I forget) Lv. 99 Aurora (Girl), with every technique, skill and almost full Lil' Cactus entry log, save for many faerie arc entries. The tempering/monster raising/golem creation stats weren't anything to write home about though. Blackpearl accessible on this playthrough.

Valkyrie Profile - Lv. 99 Valkyrie (only) and Iseria Queen beaten exactly 12/13 times. Working on amassing Dragoon Faith helmets and beating Hamsters. All characters and several complete voice collections.

Those two I backed up on my PS2 memory cards to forever have access to them in the rare case that I come across those games in my lifetime again.

My PSP precious save data is Grand Theft Auto: Liberty City Stories for emulation purposes and all the SNES rpgs I've stored along with save data. Those are easily acquired and backed up, but without the GTA save it'd be obviously pointless.

Probably FF7. It was a perfect file. After it got deleted, I never spent time mastering other games.

My Pokemon Yellow comes to mind immediately along with PSO.

600 or so hours on Yellow and I went through all the Pokemon maxed level.
PSO is on my 800+ hour mark and I'm on the Caves 2 of Ultimate Mode. I give it another 400 hours before finishing Ultimate.

Actually recently I was playing with the games on my GB mem card and accidently erased the whole card. I was so pissed. I had all my gameboy saved data there (beat games, games I was playing with cheats, games I was playing without cheating, my first ever files). So I know how it feels to lose valued data. So now I don't hava any. My PS2 data is easily reattainable :p FF isn't that hard (until I finally beat X-2)

Zelda 64 - the first game I had ever 100%'ed
FFX - first RPG I had ever maxed out
Brain Age - tracks lots of stats
My Brain - without it, I'd go something something

I'd have to say my Final Fantasy X data is a pretty solid save. I have Tidus, Yuna, and Auron as my three main characters, and each other them have gone around the sphere grid completely. They are unstoppable, which makes the game way too easy, but still, it was fun to overdose them all on power.

I have a few Xenogears saves before some key or just cool plot points. It's a long game, so I don't ever feel like replaying; I just use my valued game saves.:biggrin:

A Mischief Makers file that I started when I was 12. It took me until I was about 16 to get all of the gold gems, and the first 2 worlds so far all have S times...I would cry if I lost that file.

And a Super Metroid speed run file I have...doesn't come close to Red Scarlet's 100% run time, mine is only 1:01 :( But it's the best one I've done so I wanna keep that for sure.

I guess it used to be my Suikoden 2 save. I worked really hard to beat that game and unlock some of the secrets. I say used to because I think somewhere down the line it got erased one way or another...ah well.

I know my brother's is his FFVII save. He's got everything done, everything unlocked and found (or just about) and is at about 100+ hours.

Besides the obvious saves, personally I'd weep bitter emo tears if I lost either my Final Fantasy X or Dark Cloud 2 save.

On FFX, not so much for the game, but I put so much effort into recruiting and creating the absolutely perfect Blitzball team that I'd be pissed if 150+ hours went to waste because of a bad memory card. Too much time and trouble getting everything the way I wanted it.

On Dark Cloud 2, there's a place you have to rebuild called Veniccio. It's a coastal place, and you have only a few minor things to place and you're done. If you do it right though, you have a few thousand creation points left over and a LOT of room to work with, so you can create essentially whatever you want.

A buddy of mine and I sat down and created this huge other portion and took up the whole map, and it looks really awesome. Again, I'd be pissed off if that work just up and vanished.

While still on DC2, I'd be at least a little peeved if my save data regarding the fish races was lost. You can build up a fish to make it an excellent swimmer by feeding it certain items to boost its stats, but you can only feed it every now and then. Well, given that I fed it a lot of stat-boosting food, it obviously took a great deal of time. And then there's to fish weighing contest, and that just took a long time to find a few good size fish.

Mine would hands down be my Final Fantasy 7 saves. I put in countless hours on each. I got like 5 games with over 100 hours each, and damn did i love every minute of it!!!

Prob Shin Megami Tensei Nocturne (lucifer's call), true demon ending,all demons, strong demons with exceptional skills.

In the past i had alot like FFX (everything completed,dark aeons,...) FF9,...but meh it just sits there,there's no point in keeping those.