I tired to make 2 puns in one sentence. Clearly, nobody caught either. 
Actually, I caught them. 
I was rolling my eyes at your post. 
Toy Story: Slinky. His humor, his voice…he supplemented so many memorable one-liners in the movie.
A Bug’s Life: Rosey is a fun character, her and Heimlich. She’s voiced by Bonnie Hunt, so I’m biased.
Toy Story 2: Bullseye. I love his design, and how Andy’s name gets written on his hooves at the end.
Monsters, Inc.: I kept feeling really bad for Mr. Bile…
Finding Nemo: I have a fondness for Peach, and her motherly attitude towards the rest of the Tank Gang.
The Incredibles: Edna is a genius, 'nuff said. I like Frozone too.
Cars: Call me strange, but it’s definitely Fillmore.
Ratatouille: Emile, even if he is a big dope, is just so lovable
WALL-E: M-O is great, I love how he ends up tagging along with WALL-E and EVE even though WALL-E is constantly unclean.
Up: Kevin is a beautiful bird…her colors are so pretty. I feel so bad during the scene where Alpha bites onto her leg. 
Toy Story 3: Dolly!
Cars 2: I reaaaally like Holley. Emily Mortimer is a plus 
Toy Story: Slinky! I love him.
A Bug’s Life: Molt or Dot
Toy Story 2: Jessie
Monsters Inc: Boo or possibly Roz
Finding Nemo: Dory
The Incredibles: Frozone
Cars: Mater or Doc
Ratatouille: Emile
WALL-E: MO (although most of his scenes make my OCD go crazy for some reason)
Up: It’s a three way tie between Russell, Dug and Kevin. I can’t pick. 
Toy Story 3: Bonnie! She’s so adorable!
Cars 2: Finn McMissile. Is it okay that I have a crush on a British cartoon car?
Toy Story: Mr. Potato Head
A Bug’s Life: Dot
Toy Story 2: Mr. Potato Head! Or Bullseye
Monsters, Inc: Mike!
Finding Nemo: Dory
The Incredibles: Frozone or Violet
Cars: Mater
Ratatouille: Emile
Wall-E: M-O
Up: Dug <3 But I love Kevin…I can’t pick!
Toy Story 3: Buttercup
Cars 2: like Virginia said, Lightning He wasn’t in it enough and I LOVE him
Toy Story: Slinky
A Bug’s Life: Dot
Toy Story 2: Slinky
Monsters, Inc: Sully
Finding Nemo: Gill or Dory
The Incredibles: Frozone or Dash
Cars: Doc
Ratatouille: Emile
Wall-E: Captain
Up: Russell
Toy Story 3: Slinky
Cars 2 Finn
Toy Story: Slinky Dog
A Bug’s Life: Dot or Francis
Toy Story 2: Bullseye
Monsters, Inc.: Not a sidekick but I do like Randall
Finding Nemo: Dory
The Incredibles: Edna Moddeeee!
Cars: Mater
Ratatouille: Django! But Skinner is my favourite.
WALL-E: M-O
Up: Dug
Toy Story 3: Chuckles
Cars 2: Holley
Toy story: Slinky dog
Bug’s life: Heimlich
Toy story 2: Rex
Monsters inc.:Boo
Finding nemo: Dory
The incredibles: frozone
Cars: Mack
Wall.e: Mo
Up: Dug
Toy story 3:Mr potato head
Heres my list.
Toy Story: Potato Head
A Bug’s Life: Dot
Toy Story 2: Bullseye
Monster’s Inc: Mike
Finding Nemo: Gill
The Incredibles: Frozone
Cars: Doc Hudson
Ratatouille: Linguini
Wall-E: M-O
Up: Dug
Toy Story 3: Chuckles
Cars 2: Finn McMissle
My favorite sidekick is MO, no question! He is adorable, and faithful.
Cool lists everyone! 
My turn! 
Toy Story: Slinky
A Bug’s Life: Dot
Toy Story 2: Slinky
Monsters, Inc.: Mike
Finding Nemo: Dory! What would Marlin do without her?
The Incredibles: Edna Mode. She is sooo funny.
Cars: Mater or Guido
Ratatouille: Emile
WALL-E: M-O
Up: Dug
Toy Story 3: Ehhh… Slinky.
Cars 2: Call me weird, but Sarge. He had about 3 lines, but he had great moments.
I agree dude! Edna Mode is hilarious!
Now that Brave has been released, I thought it was time to revive this thread.
WheezysBestBud: You’re about right. And I’m suprised I’ve never posted here!
So… here’s my list:
Toy Story:Hamm, Rex, an Slinky
A Bug’s Life: I haven’t seen this movie in a looooooooooong time. Maybe I’ll say the Circus bugs.
Toy Story 2: Jessie and Bullseye
Monsters, Inc.: Celia.
Finding Nemo: Dory… Enough said. And I’ll also add the Tank Gang in there.
The Incredibles: Frozone.
Cars: Mater. He’s just so funny and lovable.
Ratatouille: Colette, because she’s no-nonsense and tough. I’ll also say the other chefs because of their wild backstories.
WALL•E: M-O and the Captain are both good choices… but I gotta say the Reject-bots (seriously, they need more love).
Up: Russell and Dug
Toy Story 3: Bonnie’s Toys!!!
Cars 2: Holley Shiftwell
Brave: The Triplets (sooooo adorable!)
Hands down, the Triplets were my favorite sidekicks for Brave.
Well, that is just too hard for me to narrow down! Let’s see, …a few honorable mentions, perhaps?
Toy Story: Misc. toys, particularly RC
A Bug’s Life: HEIMLICH!
Toy Story 2: WHEEZY!
Monsters, Inc.: Um…BOO!
Finding Nemo: Yeah, it’s between the tank gang, Crush and the turtles, and Bruce and the sharks…
The Incredibles: Edna Mode, darling.
Cars: Mater? Or maybe Luigi and Guido. And I love Ramone, Sarge, and Fillmore!
Ratatouille: Emile
WALL*E: M-O and Hal
Up: SQUIRREL! I mean…DUG!
Toy Story 3: Ken. hands down. Can you say “Cool and Groovy!”?
Cars 2: Finn McMissile, British Intelligence.
Brave: Hamish, Hubert, and Harris. And unless he’s too big of a main character…Fergus.
Whoa, I think I just went crazy with this post. Just such beloved characters from all these movies!
My top favorite is M-O. No question about it, he’s so adorable! No wonder he always seems to win…
Hamm, Mr. Pricklepants & Dug.
i think its jessie and buzz, by a tie. bullseye is a close second though.