10mans I'd give you that, though I think some of the fights in the aforementioned raid zones are the reason Wotlk Naxx, Sarth, Maly, etc seem so easy.
Yogg 1 is skill based, really the only RNG is who will be targeted to be constricted, malady of the mind, etc during phase2 and where your crushers will spawn in relation to the other tentacles (tab-dotting vs burn one and run to the next). The skill bar isn't exactly high, but without proper DPS inside Yogg's brain the people outside the brain can get overrun. Managing sanity while avoiding all of the shit that can make you lose sanity over the course of the encounter isn't something any random person can do.
Heroic: Firefighter is also somewhat skill-based, though it's much moreso in reaction times and situational awareness than knowledge of your class or pushing for max personal DPS. There's a little RNG with where frost bomb drops, if rockets are on melee or if a ranged simply can strafe 5 yds away from it, or which direction spin-up starts on dictating who needs to move and who can mindlessly DPS/heal for a few seconds. There's a lot of mechanics to the fight that can 1shot nearly any non-tank, but learning the cheesed down 10man version can prepare you for the more challenging hardmode.
Hardmode Freya is currently the hardest encounter I've learned in the game, though while my guild was learning M'uru I was raiding and dicking around on Onyxia server with Adrenaline's guild and judging by our guild strat forum that would probably take the cake. I transferred back 2 weeks before they got the M'uru kill and hadn't been worked back into raids until the 2nd or 3rd kill. Freya has RNG but there's A LOT more situational awareness and knowing what to do depending on which set of adds is spawned than said RNG. That said, if you go to phase2 cleanly and have a solid strat for bombs/roots/trees the fight almost instantly becomes easy as shit. This was a 1shot for 10man my first time in (sadly, that was this week), but took 3-4 days to get a kill in 25man even with 3/4 or more of the raid having killed it 10man.
Original Gruul was rough, as was the first Kel'Thuzad kill we got, but those 3 hardmodes are indeed difficult. It doesn't take nearly as much effort to get repeat kills on those 3 hardmodes as the first kill (1-2shots of them aren't uncommon), but the same could be said of original Gruul and KT.
