"Jo, how could you say GU's HBG is the worst HBG when Valor exists?"
There is an important distinction to make right off the bat: I don't think GU's HBG is the worst in terms of DPS, that accolade goes to Frontier. I think GU's HBG is the "worst" in that it's the least fun and skillful version of the weapon. I'm going to talk about each of the styles, each of the hunter arts, the Shield attachment, and Internal Ammo. Let's start in that order
GUILD STYLE
Guild style is alright. It's basically identical to 4U's HBG. It's outclassed by Valor and Adept in terms of survivability and by Aerial in terms of fun. Why not just play 4U's HBG instead if you wanted to play this style of HBG?
STRIKER
This is a pathetic replacement for the Limiter Removal. You do not get a raw boost to account for the lack of Sieging, so you basically just handicap your own damage by playing this style. Oh, you get 3 hunter arts? Cool. HBG Hunter arts suck, but we'll get to that. If you want to use Striker you'll be forced to invest in survivability which means either 2 of those HA slots go to Absolute Evades, or you use up precious skill points on Evasion+. Just to add insult to injury LBG gets to have the best of both worlds since Full House lets you reload every shot like LBG's Limiter Removed brother and you get to keep Rapid Fire! Now to pour lemon juice on the injury Alchemy Style HBG is just objectively better than Striker. This is not only the worst HBG style, but possibly one of the worst styles in the game.
AERIAL
Now we're cookin! This is my favorite style. It's fun and flashy, and introduces a great high-risk high-reward movement tool. However, Adept dodge is objectively better in the same role as the aerial hop, and it's easier to use. You are also of course dealing less damage than Valor. Realistically there's not much reason to actually use Aerial.
ADEPT
Adept style is really unfortunate. It's one of the most effective HBG styles, but everything it does Valor does better. Adept doesn't have anything fun or unique about it.
VALOR
Speaking of Valor, here it is. Valor style is basically everything a HBG player could ever want. Insane mobility, free evasion+2, and increased firepower! Well, now that HBG has all these things it's a boring, repetitive weapon that's extremely safe and you practically aren't even fighting the monster anymore. No need to look for openings, barely a thought heeded to positioning, just pew pew pew and the hunt's over in about 3 minutes. HBG gained everything at the cost of it's soul.
ALCHEMY
The contrast between Valor and Alchemy is honestly funny considering Valor is one of the most broken things the series has ever seen and Alchemy is just barely better than Striker. Alchemy can't siege just like Striker, but it's big upside over Striker is... drum roll please... free reload speed +1
. OK. Still no raw boost. Still have access to 3 hunter arts and no survivability. You'll want to be shooting as often as possible to make up for the significantly lower damage you'll be doing and this makes the Alchemy Barrel not really worth using, especially with HBG's long unsheathe animations. At the end of the day this is just Striker with Reload Speed +1 and an Alchemy Barrel that you might be better off ignoring. Now onto the hunting arts.MASS COMBINER
this isn't even an HBG hunter art but it's worth discussing. This is the most commonly used HA with HBG, especially with Valor style. What it does is guarantee combo success and it ensures that you'll recieve the maximum amount you can from the combo. It turns you into a hyper-efficient ammo factory. It saves resources. The fact that HBG hunter arts lose out to a glorified combo book should give you a hint to how effective they are.
GUNS BLAZING
This hunter art is completely redundant. It increases movement speed, and reduces reload time and recoil. Adept, Valor, and Aerial don't need the movement speed boost. reload speed isn't that big a deal with how siege-focused HBG is in this game, and if you actually needed the recoil reduction you would get recoil down armor skill instead of this HA that takes forever to charge. Even on Striker and Alchemy this is a subpar HA.
GUNPOWDER INFUSION
This hunter art makes your next 22 shots have 10% more damage. If you're using something as hyper-optimized as Valor then the 4 seconds it takes to use this hunter art actually causes you to lose DPS. This HA is fine with Striker and Alchemy but relying on a hunter art to get a fraction of the raw boost from Limiter Removal leaves a bad taste in my mouth.
VOID PIERCER
I forgot this was in the game. It deals a decent amount of cutting damage but requires you to be in melee range to pull it off which make it annoying and risky to use. If you really need to cut the tail and refuse to use anything other than HBG just use slicing ammo.
SUPER NOVA
this HA is barely worth the time it takes to charge up. It's got cool factor and that's the best thing about it. Who doesn't want to shoot an atom bomb out of their HBG? people who actually care about optimization, that's who.
SHIELD ATTACHMENT
For the first time ever the HBG shield is affected by Guard+2! Hooray! Maybe the shield attachment will finally be worth using! Oh, Limiter Removal is gone, and so is the improved shield attachment that came with it. I feel personally insulted. HBG shield with Guard+2 in GU is WORSE than the base Limiter Removal shield from 4U. I am baffled and thoroughly frustrated.
INTERNAL AMMO
Most of the internal ammo is completely useless. Cannon, Heavy, Shrapnel, Force, ETC. Dazzling was good in Generations but they gutted it by giving Dazzling guns less than a dozen shots total in GU. Slicing ammo is actually pretty good, I'll give it that. Also Flash Ammo is really cool even if it's unwieldy. Piercing Elemental shots and Elemental Lv2 shots are both internal ammo so I hope you didn't want to use elemental HBG on a multi-monster hunt because being unable to craft more internal ammo makes this borderline impossible. The majority of internal ammunition is just bloat filling up your ammo wheel but most people only ever use Pierce S Lv1 anyway so it isn't that big a deal
IN CONCLUSION
Monster Hunter Generations Ultimate's iteration of the HBG completely crippled the non-siege playstyle, and Valor Style's unbridled power overshadows HBG's other styles which would be lackluster even in Valor's absence. Forgettable Hunter Arts and a few confusing decisions like the inclusion of Internal Ammo and the Shield Attachment's changes only further make GU's HBG, in my eyes, the worst iteration of HBG.
