A tie between Ghirahim and Groose. Ghirahim may have been reminiscent of a lot of villains in anime, but that's not necessarily a BAD thing. I love bats**t insane characters, as well as creepy ones, and Ghirahim nails both of those attributes. Plus, he's genuinely hilarious with his overexaggerated motions and obsession with himself.
Groose, on the other hand, starts out as a character that's extremely easy to hate (for the right reasons), but is laughably controllable by Zelda because of his huge crush on her, creating a balancing offset. He's also a huge coward at first. Later on, however, he becomes an extremely likable guy and a total badass -- seriously, he built the Groosenator all by himself and used it to help Link out, and he stood up to Ghirahim! Sure, he got his tail handed to him with one swift kick, but he didn't cower in fear like he would have in the beginning.
Also, both characters' themes are awesome.
Actually I agree with Cassandra that most of the characters felt lame. Didn't like Ghirahim either, especially with the comments about Link being such a soft boy, felt like a pedophile to me.
Cassandra's actually joking. She frequently says the opposite of what she actually means -- she's often said that Ocarina of Time sucks, lol.
Also, when did Ghirahim ever say that Link is a "soft boy"? And, really, a pedophile? Seriously?
Fi because she's by far the best companion Link's had in terms of actually being helpful.
Helpful, but insanely annoying. *BREEN* *BREEN* *BREEN*
Still, she actually beat out Midna as my favorite helper -- focus on the word HELPER. I don't think she's a better character.