Ominous Woods are my favorite. As a technomancer that inevitably favors Thaumcraft in every playthrough (I cannot escape Azanor's grasp!), I find the ominous woods to be exciting. I love the color, I love the danger, and I love that it counts as a magic biome for all of my thaumotological needs.
Other places of note are
Deadlands: extremely dangerous, plenty of lava, cool color.
Icy Hills: I love ice sculptures.
Silkglades: I love the dull colors and atmosphere here. The whole thing just feels "serious" to me. I don't know why. The free string is pretty cool as well.
Coral Reef: Because it is something to do in oceans.
Also, the Promised Land as a whole. It is perfect for a fantasy setting, since you can make gigantic floating geometric crystals there, and no one will bother going "Ewww... it's floating. That's not realistic!" because the whole biome is floating.