Listing the "most frequent chord" is a weird analysis, I'm more interested in the "most frequent key", or a transition matrix from one key to another, e.g., if I'm in F, what's the chance I go a fifth up to C, or a fourth down to Bb. Just telling me G is a popular chord doesn't do much.

Exactly, this is useless. It's like saying the letter E is the most used letter in the world and Wheel of Fortune is your dataset.

The "haunting" riff in the Hounds song features a tritone, and it's a modal-ish progression - perhaps with hints of folk music lurking in the background.

You're not going to understand it by counting chords.

A lot of pop has these quirks. Even things that sound like I-IV-V or I-V-vi-IV bubble gum.

Slapping labels on the most obvious chords in a naive way misses them completely.

G is the best one though, maybe D.