Ambiguous chord progression Am-C-Em-G (vi - I - iii - V, or ii - IV - vi - I?)

There is this chord progression (used in Undisclosed Desires - Muse, Comptine d’une Autre Été - Yann Tiersen, What Is Love - Haddaway, Uberlin - R.E.M., Enjoy the Silence - Depeche Mode…) of which I don’t know in which key it is written. Let’s take an example: Am-C-Em-G. Is this vi - I - iii - V, or ii - IV - vi - I? The first progression is in C, the second is in G. Maybe you can find out which one it is by listening to the melodies, whether the C note is emphasized or the G note.

Given that it resolves back to Am each time, I’d say it’s in A minor (the relative minor of C major)

Actually, I take that back; it depends on context. Listening to a couple of your examples, in What is Love the first chord is the most emphasized, helped by the fact that the melody has a G at that point (during the Gm chord), but in Enjoy the Silence the strongest pull seems to be toward the third chord. This might be due to his singing the dominant at the same time, so it sounds more like a resolution. Also, the fourth chord is inverted, reducing the sense of it pulling back to the first one, and making the VI - i more prominent. Either way I think this progression is in minor, not major, as the emphasis is definitely on the first and third chords, not the second or fourth.

I don’t know, maybe your ears can help?
In C:
In G:

The difference is the F (in C) and the F# (in G). Which one do you think that sounds better?

I figured out the chords of Enjoy the Silence: the verse goes like this: Cm-Eb-Cm-Eb (repeating)
When the vocals begin:Cm-Ebm(!)-Ab-Ab (repeat). Then there is the chorus: Fm-Ab-Cm-Eb64. And the chorus ends with a B. Interesting song by the way! Let’s suppose the verse is i-III (because the progression repeating for so long, without using i or III, would be very unlikely), then this is the relative chord progression: i - III - i - III… i - iii - VI - VI … ii - VI - i - III64 … #VII

  1. This iii is really weird, I can’t determine its harmonical function.
  2. If I were to analyse this whole song with the Fm as tonic: v - VII - v - VII … v - vii - III - III … i - III - v - VII64 … #IV.
    Does that make more sense?
  3. The #VII/#IV chord gives no clue either, it’s out of its place in both progressions.
  4. I agree with the fact that the inversion of the Eb “weakens” the cadence. If it were like this: vi - I (Cm-Eb), the Eb should be emphasized because it’s the parallel of Cminor, but the inversion “blurs” the transition… Does that indicate that the last two chords are not i - III, because that cadence isn’t emphasized? If this is true, the song is in Fm.

Someone already made an analysis: