Hmm... I'll do a couple here. Keep in mind that I haven't bought any cards since about 2000. I'll pick a few from my collection. (so no ancestral recall or stuff like that; I didn't quite get into magic early enough for that)
Anyone here remember 'black summer'? I -still- have a variant of that deck kicking around, and it -still- wins, even with this being restricted.
I cannot possibly count the number of otherwise unwinnable games this card has managed to turn around for me. Its low cost and versatility as well as its sweeping board effects make it probably the best white card ever printed up to when I stopped buying cards.
I love control decks, and this card loves them, too. Tied with Mana Drain as the ultimate counterspell, but I don't own any mana drains. The most fun I've had with this card is against n00bs who'd never seen pitch spells before :p But, but, you're tapped out, what the heck do you think you're doing? Surprise!
Dual lands
Love them! This is the only one I own more than one of (I have four of it, in the necro variant I mentioned above. Actually, Balance is in that deck, too)
The only reason this card isn't worth as much as a mox is because it's uncommon (wtf?!?!? :p) and because it survived one more set than the moxes. There are very few cards that are more welcome to see than this in your opening hand.
I'll list Tinker as an honourable mention :p Urza's block was so ridiculous, and this was one of the best cards in the set, at uncommon, if you can believe it. MTG was already starting to die out amongst the crowd I hung out with, due to the increasing speed at which expansions were printed, and after the urza's block, most people I knew quit. I lasted a few more sets, but it got ridiculous with new expansions coming out faster and faster, as often as every 2-3 months by the time I stopped buying. From what I understand, they're still pumping out 4-5 sets a year. Magic is a great game, but it turned into too much of a money grab for me, but I still love to play with what I've got when I can find opponents. They did the same thing to AD&D when Wizards acquired TSR, too, and Hasbro buying up Wizards made it even worse, but I'd better shut up before this turns into an epic rant :p