For Portsmouth fans, would there be an edge to playing Argyle comparative to games with Brighton, let alone Southampton? I struggle to believe it. Surely, a rivalry has to be vaguely reciprocal?
With Exeter, it feels a bit manufactured. Again, to be a proper rivalry there has to be some element of competition and they've always been a smaller club. Man United and Man City, I suppose, have a proper rivalry that one team has consistently dominated, but nevertheless the Manchester derby lacks the stature of Milan/Inter, Rangers/Celtic, Barcelona/Madrid etc. probably because it's a bit one-sided.
For your essay, you might want to look at the impact of the internet. We probably wouldn't describe Cardiff or the Bristol clubs as our 'rivals' because there are n't sets of tiresome supporters calling each other "the scum" on facebook and we and they have other official 'rivals'. It's these clubs (and Swansea, to a lesser extent) whom I would vote for in your poll, though, because at the actual games there's always a bigger away following than usual; a heightened police presence; and a more raucous atmosphere that makes victory all the more enjoyable and defeat more infuriating. Plus we've played those teams pretty regularly over the 20 years that I've been supporting Argyle, indicating that as well as relative geographical proximity, they're also quite similar-sized clubs.