On two very short lists for me, one being genuinely unique conceptually/mechanically and the other being the newest member of my go-to party games list.

Some maps are definitely better than others (with me and my friends generally preferring the not so vertical ones) and some weapons are also better than others (stock shotgun/blunderbust(?) is all around excellent, horse jockey(?) shreds hallway infested maps (as well as lingering railgun) and the candelabra seems top tier at KOTH as a mode in general.

It's very rare that I'm just taken aback by how ingenious the core design of a game is; if the main factor of models being invisible was taken away, this would be a mediocre quake clone at best, but it's the simple act of making models invisible (and not so simple act of solid map design with brilliantly easy to understand color coding)

The only downside of the core mechanic(gameplay design?) to literally screen peek / screen cheat is that some players find disorienting and very mentally draining, due to effectively keeping a constant eye on the entire lobby (in this case, 3 other povs) while maneuvering yourself and trying to engage in combat.