I've always liked Monster Fighters and Atlantis.

Wow this is hard. PM, SPIII, and Agents are all pretty close for me.

I'm not sure either. I'm torn between Monster Fighters and Atlantis.

I like the sets and Minifigures from Atlantis better, but I like the Storyline of Monster Fighters better. The Moonstones are much cooler then the Atlantis Treasure and Keys.

Hard I like ATNTS, PM, Agents, and MF

So far, Agents has the highest number of votes.

I always thought Power Miners had awesome sets with really good play value. If I remember correctly, they were even some of my first few System sets that weren't Creator Basic.