Don't use any famous actors, especially comedians, like Robin Williams, Jim Carrey, etc. If you choose a sound clip that's well known then people will think about that movie and the actors that say the line, not the character that you are animating. They already have an image of how the character should act, based on the real life actor. You don't want your character being compared to the voice actor, you character needs to stand on its own.
So find something from an old movie, or just obscure movies, or actors that are not well known, something that hasn't been used before. I also wouldn't use a clip from an animated movie or tv show. If you do pick a famous actor then pick a less famous movie he or she has been in.
The clip should be in English. Unless the animation is super sweet, people will get put off by something they don't understand and subtitles won't make it better because they will distract the viewer from your animation. If you have been working in a different country, then that's another story, but now you have the choice, so choose wisely.
I'm writing this from the point of view if you're in school here in the US. I remember having audio tossed out by teachers because of foreign languages and hard to understand accents.
Obviously the animation should always count, but if you're applying at a company that does movies in English, and you have a choice in your sound clips, then I would choose an English one.
The clip can't have profanity in it, or anything that's racially or religiously demeaning. You're trying to find a job, so offending the people at the place you're applying is not the right move. Same goes for anything sexual. It might be funny to your friends, but the recruiter might think differently.
Use something that has some contrast in it. A clip in which someone screams for 10 seconds is not a good choice. If during the first 7 seconds he/she mumbles and THEN screams, that's better, because there is contrast, there is a change, the character is going somewhere with the performance.
You need to show that the character is thinking, a thought process must be visible. You can do that by an emotion change, which is a popular assignment to hand out. Present the character with a choice he/she has to make, face him/her with a conflict. I'm also a fan of pauses in dialogue, which can you give the opportunity to have your character think, react or change the tone of the clip.
Don't use any clip that has exposition in it. That means a character explaining something, like an evil plan or something like that. It's like reading a manual, there's not a lot of thinking involved, or conflict, or contrast. Unless you want to play up the contrast between someone saying something really boring while the action is happening in the background, like in this clip:
Hope that helps!