My old PS2 memory card. It was an Action Replay Max one, I think 4x the size of the official ones (64MB or something?) and I kept ALL my favourite games on it. Every GTA, Final Fantasy X, X-2 and XII, Burnout 2, 3 and Revenge, plus about 60 others.
In the GTAs I had a combined 30 or so files, including a 100% complete file of all 5 (III, VC, SA, LCS and VCS), and several saves just before my favourite missions in each game. In Vice City, I even had 3 100% files, all done separately from scratch. FFX, I had an almost-100% complete file, by which I mean all I had left was to get my stats up from ~200 to 255, and beat the final 4 Dark Aeons, Nemesis and Penance. I had the Ultimate weapons, loadsa money, had beaten all the Monster Arena monsters (besides Nemesis and Penance) and the rewards from every sidequest possible. X-2, I had virtually all the abilities from almost every... thing, I can't even remember what they were any more. FFXII, again, nearly completed, all I needed was a bit of powerlevelling to beat Yiasmat and that robot thing (Omega XIII?) Every Burnout was 100% complete. And I played The Warriors a lot - although it takes about 5-6 hours to complete and 15 or so to 100%, I had a >200-hour save on there. I had not used hacks to do any of this, but I had done to mess around on GTA and a couple of others.
This amounted to well over 5000 hours of games, about 75% of my PS2 gaming time, and about 30% of my time overall. The memory card just decided one day that it needed to format itself, which it normally does as soon as you get it. Without doing this, every time I tried to access a save, either the saves didn't show up, I got an 'unformatted card' message or it just froze. I couldn't even salvage the card, since the formatting failed every time I tried to do it. I think that was the last time I played my PS2.