I think they should have shown exactly why Remy chose to make ratatouille for Ego. Maybe he discovered some back story to Ego which inspired his decision?
I love the movie, but this part just seemed kind of … random and deus ex machine-ish. Why on earth did Remy pick a peasant dish in the final showdown? How did he know that it would have such a big effect?
Please post any ideas or rebuttals! 
Eh, I understand what you mean, how that would have made sense. Honestly, as common, and perfectly acceptable, as that is with films I think that I prefer the absence of reason over being cliche in this particular case. I don’t think Remy knew it would have that sort of effect, exactly. I think he was simply confident in his skill that it would be good. But I understand what you mean when you say why would he choose to serve a peasant dish to a food critic with so much on the line. It was a little strange but I’m personally alright with the vagueness. A line earlier in the movie about any sort of dish being great no matter the background or reputation couldn’t have hurt but for whatever reason it obviously wasn’t seen as necessary. -shrug- Interesting to think about though :]
Maybe he was trying to show that although it was a really average dish, he was skilled enough to make it something grand.
Yeah, I think 1986 has the right idea here. There was definitely a reason behind Remy choosing to make ratatouille, because in a way if I had to choose a dish that I think best represents the character of Remy, it would be ratatouille; most people consider it to be a peasant dish (in the same way that most people consider rats to be vermin) but if you look at it in a different way then it’s delicious (as we, the audience, are able to look at rats in a different way).
Ego’s reaction to it must’ve been purely coincidence, though. I adore that flashback into Ego’s childhood, but it has to be said that Remy was very lucky in choosing the exact dish that reminds Ego of his mother’s cooking!
I always thought he just chose it because it had the word “rat” in it. 
I guess Remy chose it to surprise Anton, or maybe to do something unexpected, like Gusteau would have done.
Ratatouille (the dish) had already been brought up - by Linguini - earlier on in the film. He mentioned that it sounded like “rat patootie”, which didn’t “sound delicious at all”. Which I think sufficiently sets the tone for the significance of the dish and its particular connection with Remy. That Remy could triumph in the end by making simple peasant fare so irresistable to those upper class gourmets is in itself a testament to his skill, but I agree with what others have been saying about the deeper symbolism, Remy being a rat, and a rat being something so readily discarded as vermin with nothing to offer. Outer labels and appearances can be deceitful.
As for the ratatouille’s effect upon Anton Ego - luck had nothing to do with it. I firmly believe that Remy would have won over the hardened critic no matter which dish he used. He was a remarkable a chef, after all. Anton’s flashback, in addition being hilariously sweet, was more of a personal revelation, IMO, since it was the first time we saw a sympathetic side to his character. Again, that whole outer appearance thing.
But then there’s no fun!! I think it was symbolic, not necessarily plot-driven. But i’d rather think about it then have it spelled out for me.
I agree with what a lot of what has been said thus far.
As previously stated, the line about “Rat-patootee” not sounding apetizing definitely builds to this moment. As well, just have others have said, the fact that ratatouille is considered a peasant dish not suited for the elite reflects how rats are considered to be disgusting mongrols not suited for anywhere but the sewers, and especially not a resteraunt.
I also really agree with the idea that Remy was confident enough in his culinary skill to make this dish amazing by emphasizing it’s peasant like flavors/ingridients.
Possibly my favorite moments in the film is Ego’s flashback. It is so cute and endearing! Remy didn’t know that Ego had this personal connection with ratatouille. He simply saw the dish as something reflective of himself, and that is why he made.
As for the nastalgic factor… Remy just got really, really lucky.
I believe that Anton’s own unexpected ratatouille connection, in addition to facilitating a happy ending, was to indicate that he too was more than he appeared. You can view it as a happy coincidence, but I think that the point of their mutual affinity for the simple peasant dish was also to suggest that the humble little chef and the ostensibly cold-hearted critic were really not so different. Deep down, they both had the same passion for delicious food. Remy’s cooking put Anton back in touch with this passion which, for a time, had clearly taken a backseat to an acquired passion for brutal criticism. Thus, I see it as quite a strong character-advancing moment (for Anton Ego).
I agree with IncredigirlVirginia on this. I believe that Remy’s decision to make Ratatouille was a symbol and a statement of the enitre theme of the whole movie. Ratatouille was a “peasant dish” that surprised a critic like Anton Ego. Remy was a rat, a vermin not welcomed in a kitchen, that surprised everyone when he displayed his passion and ability to cook.
I think the reason why he made ratatouille was to not conjure a pleasant memory. The reason why her made ratatouille was to prove that something so small, so insignificant, so unexpected can surprise, awe, dazzle, and impress.
I had always thought that Remy knew about ego’s connection with ratatouille, I see his mother in his flashback to his childhood, wearing all pink. Then think back to the old lady in the beginning of the movie where Remy and his clan lived in the ceiling. I always believed that the old lady is ego’s mother, and Remy knew because of being over them all of his life. That would also explain why Remy can cook like he does, if he lived over a food critic, they were probably cooking all the time, and he paid attention instead of just being a rat like the rest of the clan. Now it never says how old either Remy or ego are so there could be some argument there, but this Is always the way I have seen it.