Warning, long post.
TL;DR is defensive profiles aren't as easily categorized as they were in the past. Selecting the right target is a skill you need to learn in order to succeed, comparing everything to marines doesn't cut it anymore.
JNAProductions wrote:GEQ-Guard Equivalent
T3 W1 5+
MEQ-Marine Equivalent
T4 W2 3+
TEQ-Terminator Equivalent
T5 W3 2+/4++
Arguably, you could add a Gravis Equivalent, at T6 W3 3+, but that would ALSO be
GEQ, unless made into GrEQ, which just feels weird.
Tyranid Warrior Equivalent
In general gravis isn't the poster child for heavy infantry with 3 wounds that many armies have gotten access to in recent years which sits between
MEQ and
TEQ. Quite a few tactical channels use eightbounds as example, because everyone is fething terrified of those hitting their ranks. EEQ?
In general, these terms are quite dated as they don't really fit the multitude of defensive and offensive profiles of modern
40k.
But, there's reasonable discussion to be had about how far from a profile you can drift before it stops being appropriate to use the term.
A Daemonette (T3 W1 5++) feels pretty reasonable to call a GEQ. Against anything AP0, they're the same. It would behoove someone referring to them as GEQ to state that they've invulns instead of armor, but I don't think it'd be unreasonable.
I feel like you have to stay a little closer to MEQ, due to their ubiquity. Despite T4 W3 3+ being a somewhat common profile, I wouldn't consider that something to call MEQ without caveats-in no small part because the usual suspects for taking out MEQ (Plasma, Heavy Bolters, really anything that's D2) become much less effective into a W3 MEQ.
TEQ, I think, can reasonably include baseline Custodes (T6 W3 2+/4++) unless you're talking about Guard or other factions with a lot of S3. T5->T6 often doesn't matter much, like with S4, but when you're S3 and going to T6 halves your wound rate, it matters.
It think the most defining thing about those defensive profiles is what weapons are efficient at killing them.
Guardsmen, daemonettes, poxwalkers, gretchin, gaunts all die reasonably well to simple bolter fire and chainswords, despite having vastly different profiles. Cheap/secondary weapons or low strength blasts without
AP just massacre these guys.
They are different from aspect warriors, immortals, beastsnagga boyz or sisters, as they will take casualties from such weapons, but not get wiped out without some effort.
Marines, nobz, tank bustas, thunderkyn, stealth suits all fall into the
MEQ category. As you've correctly said, the most important things to wipe them out is
AP-1 or better and two damage. Adding an extra wound definitely kicks you out of this category, every thing else doesn't really matter.
Ironically, once a unit goes up to four to six wounds, all those guns great for killing
MEQ also become efficient at killing those models as well, dropping off sharply with the seventh wound.
The main difference between terminators and gravis/eightborn/wraiths is the save. 2+/4++ on three (or four!) wounds means that weight of attacks is not an option to handle that unit, and neither
d6 damage anti-tank nor overcharged plasma will easily kill this unit. If you did not bring terminator-killing weaponry, you will seriously struggle to bring them down. Meanwhile gravis equivalent units are fairly durable but can be wittled down even if you didn't bring any weapons 3 damage and decent
AP.
Your 3W T4 3+ unit definitely falls in the gravis category, weapons like rokkits or battlecannons would still splat them, but single damage weapons struggle to shifte the unit.
Note that most mounted units also fall in this category, due to dying to the same weapons.
While writing I noticed that I actually don't pay much mind to toughness when shooting infantry. I can't tell you precisely why that is the case, but unless I'm wounding on sixes, the choices I described above aren't impacted by the to wound roll. On the flip side, lethal hits rarely change whether a weapon is good at shooting a target, so there is that.
Also, I remember people bandying about a REQ (Rhino Equivalent) back in 8th and 9th. Would that be common enough to be worth naming?
Well, the rhino is more or less unusable outside of chaos legions, so it's not a good name that said, the defining parts of vehicles and monster is whether their toughness is 10 or higher (missiles and meltas wound on 5s), 12+ (most one shot-weapons are S12) and whether it has an invul or not to shrug off high
AP shots (melta). Similar as with terminators, 2+ armor decides whether they shrug off small arms and horde melee or can be ground down with weight of attacks. You could split these into different classes of vehicles and tow sizeses walkers/monsters, but you are better off checking each of these qualities against the weapons your army is equipped with.
Last, but not least, there are a few units which fit none of these categories. They regularly trip up people when they encounter them because they don't understand how to kill them.