So OK, if I were to take a stab at a tier list, I think I would actually re-sort the multi-class one quite a bit. A big thing for me is that a multiclass really should give you something better than if you had two characters single-classing the respective classes or if you just remixed some of your existing MCs in your party; in other words something truly synergistic, and as a result some classes fall off pretty hard, with a few notable exceptions. I'll mostly ignore subclasses unless something is very notable. This isn't saying the lower-tier classes are bad for MC, just that you might just be better off not MC-ing or that the lower the class is on the tier you may have to work harder to get synergistic results (maybe by mixing with a higher-tier class). Within a given tier, classes are not in any particular order, though I may group classes that are in the tier for a similar reason.
S tier:
Fighter, Monk <- extremely useful passives and abilities/buffs that are useful for all sorts of mixins. Tactical Barrage or Duality (intellect) are both great for casters for example. ironically, i would find a fighter/monk multiclass not that exciting, but a fighter/anything-else or a monk/anything-else is pretty darn easy way to get a synergistic MC.
Ranger <- a little more strictly limited to damage dealers and you have to work for it, but you can get a ton of generic accuracy and, later on, damage bonuses or defensive boosts. Stalker can get you unconditional bonus passive AR, which is really hard to come by (even a Barbarian's is limited to a few damage types). I was tempted to rank this down but to be honest it's only because you have to metagame and work for that max-ed out hunter's claw (and you can cheesily exceed the 20 max no problemo), not because it's weak in any way.
Top tier (may be situationally S-tier in the right setup, but not as general as actual S-tier)
Priest of Wael, Priest of Skaen, Trickster <- spam defensive spells for glass cannons
Shifter <- some interesting synergies with extended shape shifting, but mostly only works with martial multiclass
Psion <- the action economy benefits for a caster are wonderful, and multiclassing with a caster means unlike a SC psion you don't get completely shut out in fights with huge aoes or lots of annoying ranged enemies
Wizard <- lots of useful buff spells mixed in that provide a great deal of utility for all sorts of classes, whether you're high-deflection (wizard's double, arcane veil, mirror image), melee-ing (summoning weapons, phantom), tanking (spirit shield, bulwark, corrosive siphon, repulsive visage, flame shield), ranged (different summoned weapons, eldritch aim), or just a caster mix (infuse with vital essence, deleterious alacrity of motion, picking up tons of passives while still getting a lot of extra spell casts due to grimoires). The payoff is a little bit worse for mixing in with other caster classes, hence why it's not S-tier.
Pretty good tier (may be situationally top tier in the right setup, but not as general as actual Top tier)
Barbarian <- frenzy can be pretty useful for various setups, bonus armor & blood thirst also pretty handy. bloodlust comes very late but on damage-dealers can be spam-city. mobility of leap or wild sprint can help out mixins that are a bit squishier.
Rogue <- riposte and escape are great, but the opportunities for riposte are a little more limited. But martial damage dealers will love sneak attack.
Cipher <- bonus weapon damage is nice and you get some effects that only benefit the cipher, which means multiclassing onto a decent body can get you more than if you're just a SC cipher
Paladin -> extra defense can be nice and in certain situations can be godly, but it's pretty niche in terms of what other classes can offer and most of the stuff you really want someone else to be the paladin anyway, so that they can use exortations on you.
Nothing special tier (may be situationally pretty good in the right setup, but otherwise nothing special)
Druid, Priest -> really hard to come up with synergies outside of any subclasses mentioned above. you might still have a good time multiclassing them, but nothing special. Ancient and Priest of Magran/Berath have some edge interactions with their bonus PL/spells that can be slightly interesting. Fury unfortunately hurts itself by limiting its benefit to just druidic elements abilities.
Chanter <- energized, skald crit-hit synergy (which chanter alone is kinda bad at), sometimes you have a build that needs to be up close and personal anyway and can advantageously position chant AoEs. but most of what a chanter can do can be done just as well or better as an SC chanter, you don't necessarily get anything special by MC-ing it.