Chord Progressions

E5 (Em?), G, C, A, D, E5... are the chords to mostly the entire song until the outro sticks to D5, C5, D5 to the end. My question is: what is the key center? Assuming E5 is a stand-in for Em, we have the following options:

E: i III VI IV bVII i... or i III VI V/bVII bVII G: vi I IV ii V vi... or vi I IV V/V V C: iii V I VI II iii... or iii V I V/ii II A: v bVII bIII I IV v... D: ii IV bVII V I...

I am leaning toward G personally because it looks the most natural with Roman numeral analysis in mind. Thoughts?