Brushwagg, Urborg Panther, Lion's Eye Diamond, Illicit Auction, Zirian of the Claw.
As I looked through the set list, thses 5 cards sprung out to me as the best contenders.
Brushwagg: It's a funny art that has always made me smile. Plus, It's essentially non-lethal in combat. As one of my friends used to say when he played with him "He doesn't want to hurt anybody. He just wants to come out and play".
Urborg Panther: Spirit of the Night is one of the most famous old-time biggies of magic. However, while he may be a nice fattie, the big thing that I loved about him was this concept of sacrificing creature to search for him. Urborg Panther was the key, as it was the one with the ability to search. No other creature in Magic had this concept before. I recall trying vigorously to make a deck work to fish out my only Spirit using this triple card combo. It starts with the Panther's unique (at the time) ability.
Lion's Eye Diamond: Not any particular reason, but it gave casual players their own version of Black Lotus, which at the time had skyrocketed a few hundred dollars in the past year or so. Most of the serious players I knew scoffed at the card (not for long, though) and it was our casual table that tried to work around it. We did unsuccessfully, but had fun doing so.
Illicit Auction: Red gains creature control. What? Oh! It gains GAMBLING creature control! I like this as casual because it adds another aspect of the game. We have had gambling in the past in the form of coin flipping, but I don't think we had players give the ability of bidding something, let alone your life. I had a friend with a sick white/red life gain deck. He'd have something like 50-60 life and then Illicit Auction the best monster on the field and offer 10 life to start. It was nice.
Zirian of the Claw: C'mon, who DOENS'T like cheap dragons?
In the end, my nomination will be Urborg Panther. I found my arguement for that card most warranted. Now, Spirit of the Night will still probably be nominated by someone, but rememeber that it's THIS guy that made casual guys love him.
I nominate Kaervek's Torch. It looks degenerate with Channel, but I think that's Channel's fault, not the Torch's. Any other time, it makes a nice alternative to Fireball/Disintegrate/etc. that is less likely to just be Counterspell'd or Misdirection'd.
There's surprisingly few cards I remember from Mirage with much affection. I guess those can be summarised as Tombstone Stairwell, Forbidden Crypt, Hakim Loreweaver, and the AWESOME Floodgate. If I had to put one of those cards in a deck it would be Floodgate, and perhaps that's why I won't nominate it - it's all about winning games, not having fun.
I think I'm going to go blue though, and have Hakim Loreweaver. I remember at the time having GREAT fun with Hakim and Crown Of The Ages to recur enchantments and throw them all over the place... "ooh, a Control Magic, let me just pull that out of the graveyard and stick it on your guy...". Even today there's decks I use Hakim in when I'm playing for fun. Plus he looks the daddy.
Eh? Nothing really stood out in Mirage. It was the introduction of cheap replacements for cards that people really liked. I mean, they tried to give us cards diamonds for those of us that missed moxes and lotii, but they were really REALLY cheap (meaning expensive and lame) replacements.
I guess, if I'd have to pick one, I'd go with Amber Prison. It was like an Icy Manipulator with an attitude! It was a great way to shut down critters you didn't like.
"Lion's Eye Diamond = WotC employees eating the wrong brownies..."
1) Great flavor captured in the mechanic - one creature is turning into a different creature
2) Great picture - the freaked out bunny by Robert Bliss, one of my favorite artists
3) Great flavor text - and it goes along with the picture
4) It brought creature destruction to blue in a way that made sense for blue
5) It inspired a whole run of cards with a similar ability, such as Riptide Shapeshifter, Proteus Staff, and Reweave