I wanted to shortlist enemies which we can encounter in Backers Build 4 and their difficulty rating and order in my opinion.
Queen carapace in my opinion doesn’t change queen difficulty as heavy carapace gives her some additional armor there and only few more HP (which doesn’t change anything too much in her general pool) and no one attacks carapace in first place, and mist currently doesn’t change battlefield too much (obscuring vision is not so critical, and you definitely won’t stand in this mist because it would mean death from queen smashers). I expect mist carapace to be more dangerous in final build but for now it is what it is.
Panic and frenzy heads are also equal for me, because panic attack can be dangerous if they will make it work only against enemies, currently it is silly that every soldier and alien around siren is panicking. Frenzy on the other hand can be also quite dangerous, but currently siren usually shout it just after spawn, so enemies tend to lose that bonus before they engage our soldiers in combat, and later on she forget to use it again.
For crabmen with spitter head you can add 1.5 point to difficulty for brawlers and 1 for other. 
By difficulty from strongest to weakest
- Queen with goo launcher (w/o explosives) 10 – too tough to kill without explosives and goo can prevent you from running away (except using jet jump or slam)
- Queen with sonic and heavy head 9.5 – still too hard to kill but at least won’t stop you from running
- Chiron Mortar with digging legs 9 – super deadly if not dealt with fast
- Chiron Mortar with fast/stun/armored legs 8 – still deadly if will hit, but he is not so accurate
- Chiron Fireworm any legs 6.5 – can distract team quite significantly and in tight spaces surprise unpleasantly
- Queen any (met with explosives) 6.5 – resilient as hell, but she can’t surprise you and after few turns she should be dead without endangering you too much (of course she is primary target otherwise she can still smash most of the soldiers with one hit)
- Crabmen Gunner heavy with carapace 6.5 – my menace, they shoot from distance, and can pop up dangerous (mostly for weapons) grenade in close range, also will hold at least 1 soldier burst
- Crabmen Gunner agile with carapace 6 – can run quite nicely, but with their armaments they can stay away especially that only carapace will grant them some more protection
- Crabmen Grenadier agile with carapace 5.5 – can sneak past your overwatch and put at risk some soldiers
- Crabmen Grenadier heavy with carapace 5.5 – quite resistant and with grenade quite dangerous
- Siren Slasher fast acidic 5.5 – fastest of all aliens, with some decent melee attack and troublesome bleeding effects, and if she doesn’t reach you she can always spit acid
- Siren Injector fast acidic 5 – acid make it somewhat lethal or at least great support for other aliens, and better movement help with injecting attack, but still need to catch you in melee as she is more vulnerable
- Siren Slasher armored 4.5 – quite nice damage if she will manage to sneak on you, her armor let her withstand some fire, if she will surprise you from behind corner then this can be unpleasant meeting
- Siren Injector armored 4 – not lethal and slower but with some decent defence, you have time to shoot at her and react in case of infection by boosting willpower to save soldier, not so dangerous if you will keep soldiers away from her
- Crabmen Gunner heavy 4 – nasty, slower than on agile legs but increased pool of HP makes them little bit harder to kill
- Crabmen Gunner agile 4 – nasty, can attack from far away and in closer range you can kiss the grenade, easily relocate from place to place
- Crabmen Tank heavy with carapace 4 – quite many HP to shoot at and his gun can harass you from afar
- Crabmen Tank agile with carapace 3.5 – if will stay afar then can be annoyance, but managable
- Crabmen Brawler agile with carapace 3.5 – carapace makes him more resistant to enemy gunfire, but still he need to run to you
- Crabmen Grenadier agile 3.5 – this dude is not so pleasant one-time assault, running from quite far away, I hate those grenades, they disappear after one burst
- Crabmen Tank agile 3.5 – can create quite nasty flanking attacks, but still not so dangerous
- Crabmen Grenadier heavy 3 – can be dangerous when attacking with surprise, but still easy and slow target
- Crabmen Tank heavy 3 – can attack from distance but quite slow and without proper protection
- Crabmen Brawler agile 3 – more dangerous than heavy because he runs quite nicely, but still small threat
- Crabmen Brawler heavy with carapace 3 – still slow even if more durable again easy target
- Mindfragger 3 – sneaky little bastards, when deployed in number of 1 or 2 they are not so interesting, but when they come in numbers or in tight corridors they can be really dangerous as they can disable your soldier for one turn in case if help of your other soldiers will come instantly, and if case when they catch lonely soldier they can be tide changers (imagine your sniper shooting other soldier backs)
- Crabmen Brawler heavy 2.5 – slow and need to run to you, easily dies from flanking shots when is closer
- Chiron Goo any legs 2.5 – goo is not so dangerous unless there are too many of ranged enemies or queen (she can pass goo squares, right?)
- Fireworm 2.5 – on their own not really worth attention, but when launched by Chiron they become priority target if your guys are not in heavy armors
- Terror Sentinel 2 – when standing alone and in the open they are not really interesting but in tight curvy corridors and surrounded by many aliens they become some pain in the back, as they can decrease your soldier’s effectiveness significantly
- Mist sentinel 1.5 – currently not quite interesting, but if they will hide something in their mist like fireworms or mindfraggers or even some ranged crabmen then it can be somewhat dangerous
- Eggs of Fireworm and Mindfragger 1 – with Hatching Sentinel they can be dangerous, otherwise they need really tight corridors and niches to pose any threat
- Hatching Sentinel 1 – without eggs useless, and with them you have quite nice delay until it will get hot, it works only 1 time
- Crabmen Spawning Pool 0 – it does nothing
Do you think some of them are more or less dangerous?