I love the scene in Arrival with Clark and Chloe in the hospital, after he has rescued her from the Fortress of Solitude. Clark has just realized that Chloe knows about his powers, and the conversation they have is such a milestone in their friendship.
I think both actors did some of their best work in this quiet little scene. Clark is scared, and tentative, but he doesn't hesitate to tell Chloe the whole truth once he realizes she already knows part of his secret. Tom does a great job of showing how Clark is dazed to discover what Chloe knows; you can just see him going over their every moment together in his head, seeing everything in a new light. He even seems a little hurt that she didn't tell him that she knew. Then, when he tells her the rest, he can't quite meet her eyes, but also can't help but seek out that connection when he's finished, anxious to see her reaction. When it's apparent she accepts him completely, and that he finally has someone he can trust with whom to share his secret, he visibly relaxes, and gives her this sweet smile that just defines happiness, at least for the moment.
Allison portrays Chloe perfectly. Her natural reporter's curiosity shines through, and her barely contained excitement that she can finally talk to Clark about his powers makes her almost bounce in her hospital bed. She is obviously stunned by Clark's revelation that he's from another planet, but she is only momentarily thrown. In the next moment she embraces this new facet of Clark's, and gives him the assurance that it only makes him better. Her reaction is the embodiment of true friendship.
I know that sometimes it seems like all of the Smallville universe knows about Clark's secret, but Chloe is still the only one with whom he shares it so openly, and to whom he goes when he needs help. This scene was the beginning of that new relationship for Chlark, and I never get tired of watching it.