Hmm,
I've always been fond of the Ice Golem enemy, for it's lasers, being super tall, and serving as a factory to create my Frost Armor and Frozen Wings
But I also have a great fondness for the Clinger and World Feeder Enemies. They bring the interesting "Cursed Flame" element to the Corruption during hardmode. And I like how the Corruption enemies have many eyes. Especially the Feeder. Considering how the Cursed Flames brings a green color to the Corruption to combine with the purple of it as well. Which are among my top favorite colors.
I'll pick the World Feeder though. I always get happy to see it. Even if it does end up killing me which doesn't happen very often nowadays.