Pops telling Speed that he can come home anytime, despite Pops disagreeing with why he's leaving. This borders on Tear Jerker territory when you remember that the last thing Pops told Rex was that if he left, he was no longer welcome in the Racer home.
Racer X anytime he compliments Speed, especially the montage at the end that reveals his Secret Identity as the long-thought-dead Rex Racer.
Sparky and Speed having their last conversation between the last race.
Sparky: [I] just wanted to say thanks, for what could be the most exciting moment of my life.
Speed: Couldn't have gotten here without you.
Sparky: I'm looking forward to that cold milk.
Speed: Me, too.
Note how they're not even questioning whether or not they will win, but are instead anticipating sharing the moment together, as a family. And for all intents and purposes Sparky is a member of the family.
The very first race shows Speed racing against the memory of his brother, who set the fastest ever time on that course. As he approaches the finish line, you see the two neck-and-neck until Speed eases off the gas, allowing Rex's record to stand.