Ramp's sibling in cost reduction means The Darkness Crystal might not belong on the list, but is very welcome for a similar effect, and this card so happens to be a cool way to spend extra mana and get very real benefits
A lot of cards like this hit all players, but this example spares the self burn, which makes it infinitely better
Not only because its a great budget option, but it also has a lot of synergy with black strategies
Beside being very effective from a flavor perspective, it really opened the door to the idea of having +1/+1 counters evolve out of something less tangible
The first four sets were numbered alphabetically in English, later sets have changed that to an unknown order
Liliana can search up an extra Swamp turn after turn after turn, generating additional card advantage with every activation