Some important cards I can think of off the top of my head: The Power 9, the boon cycle (lightning bolt, dark ritual, giant growth, ancestral recall, and healing salve), Shivan Dragon, Birds of Paradise, Llanowar Elves, counterspell, wheel of fortune, and Wrath of God are all extremely iconic cards from the first sets and are referenced by the community and by other cards a lot. There are many other older cards like force of will, Necropotence, Juzam Djinn, and mana drain that are also very iconic.

Some newer cards that are very famous include the original planeswalkers from Lorwyn block, Slivers, Eldrazi, Phyrexians (especially the praetors), the Theros Gods, and characters like Urza, Nicol Bolas, Yawgmoth, Mishra, Teferi, and Karn are all very important in the magic stories, and many have multiple cards.