As an enchanter, either one can perform just as well as the other solo.
Coercer is a bit easier if you ask me. A Illusionist shouldn't lose to a coercer though, Spellshield is a huge benefit.
In Group v Group Illy has the huge advantage of not having to rely on reactives to do the majority of their damage. A good group (which there aren't many) will be quick on the cures, so your reactives won't do shit.
Both enchanters can be very powerful in PVP when played well.
Summoners, on the other hand, have a disadvantage. You have minimal crowd control (see: enchanter), no Manashield (see: sorcerer), and in general it is a lot harder to succeed. As a necromancer you have a worthless Feign Death (compared to the Conjurors stoneskins), and in my experience a lot less burst damage. Conjurors also have the nice ancient which snares the target (unbreakable) for like 55% in PVP. Summoners can -definitely- be powerful in the right hands however. (see:
Koppar)
Druids... Both Druids are very powerful in the lower tiers. Once you start getting into the 60s-80s the Warden really shines, as the Warden is a lot more defensive than the Fury. Warden has the benefit of Naturewalk, which means you (& group) can't be rooted and it's pretty hard to slow you down with snares. You also have Gale, which is essentially a group buff that roots you in place and increases your defense and provides a chance to knock back someone who hits you or your group. Combine that with all of your heals having HoTs on the end of it, Purity, AA specs giving you a total of 4 death saves (single & group will both proc twice), and elemental ward.. you'll be pretty powerful when it comes to healing. Furies can also still survive in the higher tiers, as well as heal very well, but most people go warden since it is a lot easier to be successful. There are maybe 3 or 4 good Furies in T8. Basically, warden is easier to keep people alive with.
Ranger is easymode. 'Nuff said. You'll be able to kill a lot of people even if you're not that great. If you can manage to kite effectively, then you will be very powerful. Definitely an easy class in my experience.
Most of the classes you're looking at are very powerful. SKs and Rogues are both capable of putting out some excellent damage, so if you want to go for a successful duo I'd recommend playing a healer. You could go with a chanter too, since against larger numbers than your duo you may be able to succeed with the mezzes, but later on you'll face a lot of people who have ways around mezzes and various CC effects (such as god spells or Sanctuary), so you'd probably be better off with a healer.
If you want to play a Fury, duoing with an SK or Paladin would be your best bet. SKs and Pallys both have great survivability (with SKs getting a riposte buff in endline TSO, as well as Divine Aura). The lesser defensive abilities of the Fury in the later tiers paired with the survivability of a Crusader will probably be easier than pairing a Fury with one of the rogues. Not to mention the Rogues not having much in the way of Taunting, you may end up fighting people who just ignore your friend and crush right through you.
/END Wall of Text