Which films do you consider as the most important animation films?

As for recent, i say The Incredibles.

everything about is just...good. So much feeling and storytelling. And cant forget, some of the best animation around.

Yep, I'd add those to my list, too, especially Toot, Whistle, Plunk, Boom - ah, Ward Kimball! And The Dot and the Line - great example of how minimal animation can convey emotions.
And no list would be complete without at least one Tex Avery cartoon, of course.
I have to say, though, that I liked Fantasia 2000 better than the original one. Goldberg's bits are priceless, The Sorcerer's Apprentice is in, too, and The Firebird Suite is an FX animation ORGASM! I saw it for the first time two years ago in Annecy and boy! when the firebird woke on cue to the music, that made me jump!

Get back in your hole, you've got work to do!

(How's the cold?)

Geisel and Jones also worked on Horton Hears a Who (1970), in which Jones also voices Horton.
A Horton Hears a Who movie is currently in development at Blue Sky.

I believe that Geisel and Jones first collaborated in Coming Snafu (1943): worth checking out.

And then there's always Horton Hatches the Egg (1942), although it's a Clampett film.

It depends on how you define "most important" in this context.

For me, "most important" means that it altered the history of animation, that if effected animation trends for decades to come.

But I suppose "most important" can also mean that it should have influenced other animated films. "Most important" can simply mean that it was important to you personally, whether as a professional ideal or as pure entertainment.

I'd like to append The Three Little Pigs (1933) to my list: probably the most important animated short since Steamboat Willie (1928).

The movie was phenomenally successful with audiences of the day, so much that theaters ran the cartoon for months after its debut. A number of theaters added hand-drawn "beards" to the movie posters for the cartoon as a way of indicating how long its theatrical run lasted.

Among animation historians, Three Little Pigs is considered to be the first cartoon where the characters display individual, unique personalities; as opposed to simple "good guys" and "bad guys". The straw and stick pigs are frivolous and care-free; the brick pig is cautious and earnest.

"The Three Little Pigs" was probably the most important and influential short that the Disney studios ever made. It was certainly the most popular.
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The short was influential for more than a few reasons. Disney had always wanted to instill his cartoons with real character and finally felt he had established it with the pigs. This would set the standard for what audiences would expect out of an animated character. No longer would audiences accept two-dimensional "cartoon" characters. A further level of depth was required from here on in. From a technical standpoint, it also made Technicolor a requirement. Since "Pigs" was made in Technicolor, that was now the standard.

Theater owners loved it as well. In an era where a short would normally stay for a week or two, "The Three Little Pigs" stayed on for months. One owner painted beards on the poster of the pigs; the longer the short played, the longer the beards got. In Dallas, a small riot erupted when the owner forgot to show the short as advertised. Calm was only restored when the theater owner stopped the feature film and showed the short. Feature films came and went, but "The Three Little Pigs" stayed on, sometimes even garnering lead billing over the feature film; an unheard of feat for a short cartoon.

Of all the short films produced by Disney in the 1930s, Three Little Pigs has some claims to being both the most significant and the most prescient. It is widely acclaimed as a breakthrough in character animation, leading the way for such indelible anthropomorphic creations as Bugs Bunny, Daffy Duck, Tom and Jerry, who dominated the various Hollywood animation studios from the late 1930s until the late 1950s. It also points forward to, and in some ways enables – partly due to its significant critical and commercial success, rare for a short animated film – the move into large-scale feature film production at Disney (pre-production on Snow White and the Seven Dwarfs [1937] began soon after the film's release). It also prefigures the wholesale appropriation and some would argue bowdlerisation of European culture that became a Disney trademark of its films, television programs and theme parks; popular legends and fairytales, children's classics and classical music. Three Little Pigs can also be considered more directly as an emblematic example of Disney short animation in the 1930s; exquisitely drawn and animated, jauntily scored, featuring fascinating details of character movement, but also a little light on laughs, a bit too repetitive, lacking in-depth characterisations, and morally preachy or unsophisticated in its ultimate, much commented upon “message”.

Three Little Pigs is generally regarded as the most successful short animation of all time.