My favorite biomes are:
4. Crimson: in worlds with this you can get knives that heal you and a book that reduces enemy defense, while in corruption worlds, you get a spear that explodes into eaters and a book that shoots green, generic fireballs. The Crimson has WAAAAYYY better loot than the corruption.
3. Surface Hallow, the water looks nice, rainbow slimes are cool, and not every enemy is immune to Poisoned, On Fire! and Cursed Inferno, unlike the Underground Hallow.
2. Underground Jungle, Chlorophyte is awesome, Plantera is Awesome, Golem is Awesome, Life Fruit is Awesome, the Lihzahrd Temple is Awesome, The Underground Jungle is awesome*.
1. Granite, very cool looking, all enemies here look cool and have good combat mechanics, granite itself looks great, and you can make very cool furniture out of it. Here you can even get a normally Post-Golem item.
*except for those Giant Tortoises and Moss Hornets.