I'm not a big fan of their older albums, but I love Collide, Comatose, and Awake. Music-wise, they're heavy enough to deserve headbanging without being screamy. Lyric-wise, you can always find at least one song that associates with your current mood. Love, despair, anger, depression... or feeling like a monster.
Also, they are amazing live. <3
Also-also, they are the only band I know who can make violins rock.
Really, though, I guess what it comes down to is I like them because they meet something inside me. Everyone has their own ingrained music likes, and Skillet is one of the bands that I can listen to and thoroughly enjoy.
Turakii