Toy-fecking-Story. I haven't seen a single Pixar-film I didn't like, of course. Every single one I've seen so far have been great, almost to a suspicious degree. But nothing can really match the power Toy Story hold over me, mostly because of my nostalgic connection with it.
And of course, its habit of finding new, exciting ways to traumatize me over the years. Like Buzz and his painful confrontation with reality.
My six-year-old self sends his teary reguards.
Of course, the internet is doing what it does best in attempting to ruin whatever I had with Toy Story... Whoever greenlighted that woody-doll is a soulless monster!
But beyond the unfair nostalgia-advantage, I don't think I can choose a definitive favorite. They've all got one thing in common, and that's one basic, interesting idea at the core. A rat with an affinity for fine cuisine, an old, lonely man commiting a more creative form of suicide by aid of balloons, sentient cars in a car-earth, et cetera. It's just good and honest craftsmanship at its finest.