Yume 2kki: The theme park, or as I like to call it, the Coney Island Disco Park
.Flow: I love the whole game. I can't pick one particular area out of them all.
LcdDem: The park, the flood plain, trypophobia land.
I've played most of the other fangames but I don't really have much opinion on them.
Y2N: Dancing girl, the clowns, and a bunch more I don't know the name of.
.Flow: The Kaibutsu, that underwater mechanic, the (robot?) maids.
LcdDem: Swing Alien. Also even though she doesn't quite count, Chie's mom.
Y2N: Small, Bike+Wolf, Spring, Cake, and while not an effect, the 'split in half' deal.
.Flow: Mechanical Limbs, Slime, Arm, Witch, Neon