Dead Island 2. Lots of different weapons plus they can be modified heavily. Plus testing them out on zombies is fun especially with the zombie damage system (or whatever fancy name they called it).
Resi 4 and its remake. Each gun has a lot of love and thought put into its little differences, culminating in a unique bonus that's unlocked when fully upgraded
A lot of games have gear to collect/upgrade. You could play assassin's creed odyssey, a loot fest. You can change weapons every few levels, upgrade them and constantly try out stuff. Borderlands saga obviously too. And the majority of mmos are based on the grind, so there's that too
Did some Bunker missions in Star Citizen, some weapons are loot-only like LMGs, also there are chests with subscriber-only stuff that you can get like special armor or Railguns/Snipers.
I'm looking for the best games where collecting different weapons is a big focus. I've played a lot of Destiny, Fortnite Save The World, Minecraft Dungeons, and right now I'm playing Remnant 2. For whatever reason I always enjoy games like this where I can grind for or find the hidden best weapons in games like this. So what else would be up my alley?