Okay, so now for the big Gloom! Gloom Stalker is a bit weird, because its kind of like the assassin-Ranger. It takes the Stealth aspects of Ranger and dials them up to 11.
It doesn't seem like the Gloom Stalker gets a ton of damage upgrades at third level, which is weird. Hunter gets 1d8 on every attack, Fey gets 1d4, Beastmaster gets a whole 'nother attack from their pet. Gloom Stalker gets 2d6 about 3 times a day. That's an average of 1.75 across a 12 turns of combat day. That's not a lot, though the fact its something you can focus on bigger enemies and ignore mooks is nice.
The big thing, however, is fighting in darkness. If you can extinguish all lights, you've effectively got a persistent Greater Invisibility. Advantage on attacks, disadvantage on being hit when they can locate you. That sounds pretty amazing... and it was great in 2014. Its especially fun in a party of a warlock with devil sight, shadow monk, shadow sorcerer. It doesn't work against tremorsense, truesight, blindsight, or the Devil's Sight invocation, but those are relatively rare, whereas Darkvision is not.
Gotta take a moment to ask, however. Is it still amazing in 2024? Advantage is much easier to get in 2024. Expecting to get advantage from this feature also means that your choice of weapons is now in question. There's usually a few categories of weapons Rangers use: shortbow, longbow, heavy crossbow, shortsword+scimitar, shillelagh+scimitar, rapier+shield, whip+shield. Occasionally you get Strength based Rangers, but considering that this is a stealth-focused subclass, I don't think we want to move away from Dexterity.
We don't want to use a rapier, a shortbow, nor a shortsword, since any time we use our signature ability, it effectively negates Vex mastery and means Topple is less valuable to you. Longbow / heavy crossbow is still extremely valuable, as is Shillelagh+scimitar; whips can be interesting, due to the way they proc O.A.s with Reach if you're deep in enemy territory. Overall, though, it looks like Slow is the dominant Mastery, except for one exception. Heavy crossbow has a d10 and Push vs longbow's d8 and Slow, but it requires Crossbow Expert before you can take Heavy Weapon Master; the crossbow is probably the best weapon if you can afford the feat.
Level 7 is... okay. Wisdom saves are important, but not what I'd call an exciting feature. Same with 15 - extra defense is good, even if not exciting.
Level 11 is... one of these is the clear winner over the other. An extra attack is massively better than Frightened, even if it has to be against a second target near a first. Especially if you're already effectively invisible from the level 3 feature, Greater Invisibility spell, or from Nature's Veil at level 14. How can they avoid walking closer to you or suffer from line of sight if they don't know where you are? Mass Fear directly conflicts with the themes and powers of the sub-class offered so far.
Much like the Hunter, I'm really iffy on this level again, because... what happens when you're both unseen and no monsters are next to each other, or its a solo boss fight? You effectively either can't use this level, or you have to give up on hiding and switch to trying to be the Scary Guy instead of the ambusher.
I feel like this sub-class was nerfed as a reaction against the 2014 version being OP. Understandable, but they didn't account for the fact that advantage is easy to get in 2024, so there's a second pseudo-nerf there too. This subclass can definitely be fun to play, and consistently bring the level 3 ability into play. But its damage is probably the absolute worst of the five Ranger subclasses.
(EDIT) Longbow / heavy crossbow is probably the best choice here, since you can more easily find patches of Darkness with ranged attacks than you will with being in the midst of melee, then pair it with Heavy Weapon Master. This also leaves your BA open for applying H.M., Nature's Veil, or even a simple Poison for a bit more damage. Honestly, I'm partial to putting poisoner's kit on the gloom archer here, then using a terrain control spell (like Spike Growth) for your concentration spell instead of H.M. Poison immunity is common amongst constructs, undead, and fiends, but for everything else its a bit of free extra damage. Spike Growth also pairs nicely with Slow and Push.