I don't like very many cards from this set. Mostly I just remember crappy cards and some good but unexciting cards. Also there were simply broken cards like Arcbound Ravager and obviously they're right out. One of my friends kept trying to make Death Cloud competitive (he took a Death Cloud deck to some Mirrodin Block Constructed tourmanents but usually got walked all over by red decks). It was a cool card because it was more versatile than Pox. But I've always thought of it as being a bit underpowered.
There is a card I only met once that made a huge impression on me. Casual decks that can gain a lot of life always need some sort of kill condition. There are some big ones like Serra Avatar, Test of Endurance, and Celestial Convergence. But these cards are white. Also, you have to have a lot of life. Just gaining life over time (even if you lose it) isn't enough. And so a green creature that gets bigger and stays bigger simply because you gained life does a good job of filling this niche. So I nominate Ageless Entity.