The friend who bought me the Unlimited deck had some pretty valuable cards for a while, including a Black Lotus. People would gather around the table and "ooh" and "aah" whenever he played it.
He eventually sold all his heavyweight cards as well, to get himself out of some financial trouble. So I guess they turned out to be a good investment.
The most valuable cards I ever owned were a Mox Emerald and a Time Vault. I actually won the Time Vault in a tournament--a tournament from which I should have been quickly eliminated.
I think a group of MTG players will appreciate this story. I showed up at this tournament with a Red/Black land destruction deck--only to discover that we wouldn't be playing one-on-one. We would be playing in groups instead.
Oh
shit, I thought. Land Destruction is just about the most worthless type of deck imaginable for group play. Yet somehow, I managed to scrape through into the prize round. Then the fun began.
I had come to the tournament as part of a group from a small city on the outskirts of Edmonton. Three of us made it into the final round--and made an alliance against the rest. My allies essentially used me as the tip of their spear: I ran wild, attacking everyone, while they defended me with every spell at their disposal. No one had expected to have to face a land destruction deck--after all, what kind of idiot would bring one to a group tournament? My own stupidity worked in my favour.
Finally, when we had eliminated everyone else, my two allies quickly disposed of me, and fought it out for first and second prize.
I won a Time Vault. Second prize was a Timetwister. First prize was a Time Walk. I think we earned the undying hatred of the whole Edmonton MTG community that day.