On order I'd suggest:
IB/Engi - IB/WP - IB/SL (in min/max'd roam spec) ~ basiclly IB/x, given the nature of IB; gotta luv the parry/crit/apfeed.
SM/AM - SM/WP ~ to utilize the stacking spirit resis debuffs. Some variation of SM/KotBs would work out too (in theory that is, you'd have to run some sort of ultra-gimmicky spec to allow for easy sustain), if atleast the SM had high RR to allow for a proper build.
SW/AM ~ the trololol-kite duo, even BW/AM works out just fine, considering the HD/rkd the BW provides.
WP/KotBs ~ kinda meh, but still feasible due to the easy accesible goodies on the knight.
AM/Engi ~ trololol-kite duo 2.0; the CC and sustain would be amazing, but they otherwise lack good synergies, sadly.
On destro I'd suggest:
Blorc/Choppa - BG/Choppa [both times choppa in min/max'd roam spec] ~ with BG/Choppa having an edge due to more easy accesible sustain on the BG.
SH/SHA ~ destro version of the trololol-kite duo.
CH/DoK - BG/DoK - Blorc/DoK ~ self explanatory.
SHA/Blorc ~ had some really good experiences with that setup, if not only for the roleplay aspect of git+warboss

BG/Mara ~ probably the only mpds/tank duo that can <properly> sustain itself.
SHA/Magus ~ unlike AM/Engi, the SHA can provide resis debuffs for the magus.
CH/Mara ~ probably the easiest setup to run with, and ultimately one of the most versatile.
[Abbd.: Sorc/Z ~ lacks in the CC department but allows for SM+KotBs levels of spiritdebuffs with the Zealot M1, it'd be a setup more finnicky to pull off properly in the lakes, but should be workable with dedication.]
[Abbd.: Initiative debuffs are amazing, regardless the setup/format you play in; they allow you to neglect static crit (which will chew through your rr on most classes - especially on destro crit is not easy to come by, unless you are playing with a deftard-esque monstro-debuffbot-mara/WE dedicated to +mps gear [<< extremely gimmicky] or with dedicated morale pumping pocket SHAs) and opt for other stats and stuff instead. The most easy accesible being on AM/SW/Blorc/SM/Mara.]
Generally speaking:
Everything tank/healer related works out just fine (aside from RP/X and Z/X, that'd would probably require alot more work to pull off properly compared to other setups), with certain setups having an edge due to the synergies they offer - the same goes for rdps/healer.
Mdps/tank setups can work out too, but are more vulnerable given most of the sustain is on high cooldowns, or has otherwise hard to work with drawbacks.
Healer/mdps barely works out, given one of the both will 9/10 times just flatout faceplant due to lack of CC and will certainly do so in prolonged engagements.
The only feasible rdps/tank setup is probably IB/engi - dunno what other setup could potentially provide as much of all the goodies.
You could also cheese it and run AM/AM - SHA/SHA, DoK/DoK - WP/WP lack the means to take care of rdps. IB/IB - BG/BG should work out just fine aswell, however it'd be a waste to run those classes in dups, given their buffs - especially IB is just too good of a buffbot to not be run with just about any other class.
As it comes to pure rdps, mdps or tank setups (aside the ones mentioned above) its... well, you'll burn through a metric **** of pots - your best bet are Engi/Engi or Mara/Mara in that regard.
TLDR: Its all about toolkits (CC, buffs/debuffs, sustain) and synergies - you gotta take into account the toolkits of the classes you face aswell; rkds are a pain in the butt for everything roaming related, if you are on the receiving end...
Abbd.: The most hilarious duo to play is undoubtly Engi/IB; stack avoidance, grab the keg, sit somewhere in the lakes and bait pugs.
Abbd.: The most versatile duo - on destro - would probably be Mara/X - you can build a Mara into a million things.
**All the above is in regards to roaming, SC comes down to just about any tank/dps or dps/dps setup.
Every kind of assisttain works out - run sorc/sorc or bw/bw for the lols, watch people blow up left and right. However you'd probably be best off providing both tank and heal, you could very likely carry 7/10 pug scenarios like that. Additionally you'd have no trouble whatsoever expanding your group later down the road.
E: Words and stuff.