Zero is a great sidekick he has a laser sword, a cool outfit, can shoot two shots out of his Zero Buster arm cannons, and
actually, screw this Zero is a shitty sidekick, because he's actually a lot cooler than boring ol' X. He really makes X look pretty lame in comparison, what with his selfless, rogue-ish attitude and laser sword and all. But he's loyal and respectful of X, so there's not much room to complain, even if his flowing mane of glorious hair is too magnificent for words and outshines X in almost every way.
Midna is one of the few Zelda companions to actually have an established personality, outside of yelling information at Link like we weren't already staring at the TV because that is how videogames work (lookin' at you, Fi), which already puts her above a lot of the competition. She's got her own agenda and is kinda a jerk to Link, but then again, if you were partnered up with some blonde guy who never speaks, you might give him some flak too.
Listen I didn't decide Tails would be number 8 on the list of Greatest Videogame Sidekicks of All-Time. You did. You people and your votes did this. Tails the immortal, self-sacrificing, two-tailed fox who could fly is number 8. Maybe it's the last 15 years of mediocre-to-terrible Sonic games that tarnished his legacy, but Tails is unquestionably one of the great sidekicks. He's the iconic "let your younger brother control this guy" character useful, unobtrusive, and completely disposable. The only real negative aspect of Tails was that his introduction opened the floodgate of the innumerable other terrible Sonic animal sidekicks that would follow.
Again I'm just the messenger. And the message is that there are 7 sidekicks that are better than Tails (or "Miles Prower," for all you people who actually think it's normal and not super-weird to call him that).
GLaDOS the unflinching, sociopathic, evil, cruel, condescending villain of Portal and the first half of Portal 2 went through a bit of a transformation after her rule over Aperture was ended by Wheatley in Portal 2. Reduced to being stuck-to/powered-by a potato, GLaDOS had some time to reflect on her evil ways, her relationship with Chell, and how great J.K. Simmons' voice is (okay, maybe not that last one), and ends up assisting Chell in stopping Wheatley's destruction of the facility. And when she's restored to her former seat of power, when she could end everything for Chell she saves her, sets her free, and returns her best friend to her. And she even sings a song! It's about how much she hates you, but still.
Things Garrus Has In Common With Riddick (aka Vin "The Pacifier" Diesel):
Beyond some similarities to Vin Diesel, Garrus is one hell of a sidekick: he's one of the only squad members available to Shepard in each Mass Effect game, he survives a rocket to the face, and he and Shepard have a great buddy-cop vibe going on. Can't you just picture Shepard and Garrus on the mean streets of Boston (or, what used to be Boston, before that whole "Reaper" thing), busting criminals and disobeying their commanding officer because they need to follow their own rules to clean up the scum of the city? CBS, you know where to reach me (firstname.lastname@example.org).