Clearly my favorite chord in the 21-second New Girl theme song is the iv, as in IV→iv:

Zooey Deschanel – Who’s That Girl? (short)

[C] Hey [A]girl, [F] whatcha [G] doing?
[C] Hey [A]girl , [F] where you [G] going?
[F] Who's that girl?
[Fm] Who's that girl?
It's [C]Jess!

The substitution of VI (A) for vi (Am) in the usual I→vi→IV→V pattern is nice, but I’ll take the iv any day as my favorite chord here.

In fact it’s so obviously the best chord that after season 1, they started airing it as just a 5-second version of the theme that’s only IV→iv→I, which isn’t explicitly on YouTube, but it’s literally just this:

Zooey Deschanel – Who’s That Girl? (very short)

[F] Who's that girl?
[Fm] Who's that girl?
It's [C]Jess!

If you like IV→iv, and you know I do, this is as close as we’re ever going to get to a song that’s 100% IV→iv.

Interestingly, pilot episodes are always a little rough around the edges, and the IV→iv was added after the pilot episode, because clearly what the show needed was more IV→iv. Here’s the IV→iv-less pilot theme:

Zooey Deschanel – Who’s That Girl? (pilot, no IV→iv)

The full song (longer than the TV-theme version) actually has all sorts of other good chords, including a dusting of II (D), and… wait for it… wait for it… wait a little longer… I→I7→IV→iv! Huzzah! It all comes back to I→I7→IV→iv.

Zooey Deschanel – Who’s That Girl? (full song)

My transcription below, non-diatonics highlighted as always.