All of the Lieges were great, but I don't think I can isolate only one.
The Retrace mechanic is very fun to play with.
Cackleburr is still stupid fun...
I've always looked at Endless Horizons and thought "This card is really great," though I can never bring myself to actually play it...
The idea of Figure of Destiny is one of the shining achievements of the Lorwyn/Shadowmoor block. An actual way to "level up" a single creature without committing any extras cards. I was really close to nominating this card. However, it's going to be cast aside for...
Nomination: Helix Pinnacle: Yeah, I know. It's an "I win" card. Yet, I've always found this card to be incredibly fun. Like the FoD, it gives you something to do with your extra mana you're not using. What this does that FoD doesn't is allow you to build a deck around it. Unless your opponent is playing a Tranquility-type card or World Queller, you can assume you're not going to lose possession of the card. You COULD go for the infinite mana rout, but I find it more fun to play the mana ramp and defend strategy. Either way, having such conversations as "Do you have enough counters to keep track of that?" or "Does that say '56' or '58' counters?" is an unusual change to typical games.
Ransac, cpa trash man