The most powerful and straightforward "thank you" scene that I could call to mind is the tag of "1966," when Lily expresses her undying gratitude for Nick's rescue of her family, mentioning so many of the lives that are better for Nick's intervention, and hinting at the ever-expanding waves of good from his action, such as her son. (Unfortunately for icon-making, there's nothing visual there to grab. It's Nick listening to his answering machine. Either you know what you're seeing in the still, or you don't. It doesn't suggest anything to a stranger.)
In second place, I think, is Magda thanking Nick, the scene I picked for my icon. He's just saved her life, and her gesture of gratitude includes almost forcing on him the memento of her cross necklace. She doesn't know what it means that he can hold it, but we do. (For icon-making, her action and stance seem to me to be visually suggestive of their emotional content.)
But beyond those?
- Rebecca's goodbye smile in "Dying for Fame"?
- Schanke's long "thank you for this award" speech in "Partners of the Month"?
- I thought of the gifts Nick leaves on Natalie's keyboard at the end of "Dance by the Light of the Moon," but I've always interpreted the message there as more "I'm sorry I couldn't let you in on the plan" than "thank you."
- There are Lacroix's endless demands that Nick display gratitude Nick does not feel (and the kissing the ring in DKII and IWR would actually make an excellent icon if you wanted all the fraught baggage that rides with it, but not, I think, a simple "thank you").
- Bernice thanks Nick in "If Looks Could Kill."
- Schanke tells Nick that he should thank him for his "pint of Grade-A Schanke" in DKII.
- Schanke says "thanks hon"/"thanks Patrice" over the phone.
- Nick thanks Norma and kisses her cheek, but that's when he's trying to distract her from a computer showing he has no past. (Nick also thanks Larry Merlin in that episode.)
- There are the curtain-call bows in "Love You to Death" and "Last Act;" those are definitely a kind of thank-you! And I think Marian and Tracy have an exchange in "Blackwing" that would fall into this category.
- "You should be thankful to have such good friends, Knight." / "Oh, I am."
- Nick accepting the pillbox from Natalie in "Be My Valentine"?
- Joanna at the end of "The Queen of Harps"?
Nick says "thank you" once in "Night in Question," when he doesn't have his memory. When does he say it when his memory is intact? He often kisses Janette when he might instead say thanks. With Schanke, he deflects thanks into humor. With Natalie, well, does he ever thank her for the protein shakes, green tea, tanning bed and garlic pills? Or is it all just, "I don't think you'll ever know how much you mean to me (because I will never, ever tell you)"? But that's unfair to Nick. Throughout first season, he regularly thanks technicians, assistants and uniformed officers, and in at least one episode, we see that Nick knows their names while Schanke does not (of course I'm thinking of the scene with John Kapelos's brother, but that's not the only time it happens that Nick calls an extra by name and Schanke doesn't). Is it somehow telling -- harkening back to/rebelling against his mortal life, or just centuries with Lacroix -- that most of the people Nick thanks are social/professional inferiors?
Lacroix seems to practice an equation of obligation and gratitude. Has Lacroix's eternal insistence that Nick is under obligation to him, and that he is due gratitude from Nick, poisoned Nick's entire approach to expressing and accepting gratitude? If so, how has Lacroix's same approach affected Janette?
And then there is Natalie, who appears in the rough, offhand list above the fewest times of all. Am I just not remembering, or does she rarely say "thank you," too?