Her suicide scene, and the scene where Ariadne goes into his "basement". Damn good stuff

Keep in mind that a movie, especially a movie like this, must do a number of things well in order to accomplish the goal of closing up all story arcs, including subtle ones. Nothing in the story is presented solely for character development or to understand the background and motivations, but also to set up the ultimate closer of all open loops.

1. Ariadne peeks into Cobb's psyche before the multi-level job began. This allowed her a chance to peek into Cobb's mind, memories and dream themes.

2. She was the main character who was given a CLUE as to what Cobbs used to judge reality or not (not JUST the totem). We are never shown any other characters being given knowledge that he spun it and would wait to see if it stopped spinning or not. As a comparison, we see 2 other characters' totems - Arthur's die and Ariadne' chess piece. BUT, we are never shown how they judge the totems, only what they are. In one scene, Cobbs actually tells Ariadne how he uses the totem.

3. We are shown how Arthur teaches Ariadne the trick of closing the maze, which fools the subconscious of the sleeper's into perceiving the dream as real for much longer. We're then later shown a scene where Arthur uses this trick against a subconscious security agent within a staircase BUT that is just misdirection to make us believe that's the only place it's used. In actuality, it's used again by Ariadne (as I'll explain in a bit).

4. Adriane is the ONLY character in the levels who has SEEN Cobb's 4th level. That means there is NO WAY Saito could perceive that level.

In the final scene, mirroring the initial opening scene, the main difference is that the spinning top is not focused on. Ariadne's character knew EVERYTHING that needed to be known to ensure that Cobb gained an inception. She, in the end, established the ultimate inception, even greater then the inception perpetrated by all the characters onto Fischer, and ultimately an inception of the movie audience. She got Cobbs to believe he made it back up all levels and into reality when, in fact, she designed a closed loop to cause him to believe he's shot back up to reality when in fact he's probably gone even one level deeper into limbo - a limbo where he can finally let go of his guilt and see his children. It was a way to provide him a "happy ending" if by chance he got swallowed up by his own deep psyche.

This is reaffirmed by the initial scene with Cobbs asking her to design a maze for him, and FURTHER reaffirmed by the emphasis to ensure the audience knows that Cobbs NEVER wants to know the architecture of the architect's designs. If the movie did not emphasize the importance of that, it would most definitely crush the possibility of the end having the possibility that Cobbs was deeper into limbo, let alone be ambiguous.