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, 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.: 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) 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 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 pain in the butt for everything roaming related, if you are on the receiving end...
Abbd.: The most hilarious duo play is undoubtly Engi/IB; stack avoidance, grab the keg, sit somewhere in the lakes and bait pugs.
Abbd.: The most versatile duo would probably be Mara/X - you can build a Mara in 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