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.
Card type: Instant
Casting cost: 2U
Card text: Counter target spell.
That spell's controller may draw a card.
Oracle text: Counter target spell. That spell's controller may draw a card.
Flavor text: "The druids would be better off if they spent more time in the lab and less time watching trees grow." -Pontifex, elder researcher
This card, for me, is excellent in most of my Three-color decks that have blue in them. Its all about the 1 single blue mana in its casting cost, oh sure there's a drawback but most of the time it doesn't hurt me that bad since I usually save a countermagic spell for only real powerful cards which aren't really plentiful in most decks I play against, in my opinion....
I'm nominating Bronze Bombshell. C'mon, besides the name, it's a way to use Donate without feeling all icky from its past association with Illusions of Grandeur. Or any way to exchange permanents with your opponent.
In that case, I'll nominated Eater of Days. He's a big flying fattie that appeals to most casual gamers but you have to set up your defenses right to not lose during the next two turns (or get into a losing position )
Darksteel Colossus - Mooseman Ageless Entity - Oversoul Vex - Killer Joe Sword of Fire and Ice - Ransac Eater of Days - Spiderman