Part of that is due to the older EXs and Savage being where a mechanic is first introduced, a toned down version popping up in the next xpac's dungeons/normal mode raids and enhanced remixes of the mechanic popping up in later EX and Savage fights.
You solo tank this and it's 100 times easier
Let's see... I'll go with my memories for at release, because to me, once a fight is no longer current, then it becomes exponentially easier due to gear or Echo. I'll start with closer to MSQ/Hard modes, and then the second list will be closer to 8-man Raid/Savage. I'm going to pair raid fights I find them comparable to in difficult after a tack mark.
MSQ Tier:
Garuda Ex
Ultima Weapon Ex
Leviathan Ex
Shiva Ex
Bismarck Ex
Sophia Ex
Zurvan Ex
Lakshmi Ex
Byakko Ex
Tsukuyomi Ex
Suzaku Ex
Seiryu Ex
Titania Ex
Innocence Ex
Hades Ex
Ruby Weapon Ex
SoS Ex
Raid Tier:
Titan Ex - Second Corn Turn 1(T6) - Both focus on positional control of the whole party, have cleaves etc. There's no permadeath in T6, but there are more TPK scenarios.
Ifrit Ex - Binding Coil Turn 2 - Vuln stacks necessitate tank swaps, Ifrit's are more punishing. No rot to pass in Ifrit, but party positioning is still controlled by a less severe debuff called Searing Wind.
Ramuh Ex - Second Coil Turn 3(T8) - Both could be solo tanked, Both had center rooted bosses, and both prevented party clumping with damaging AOEs. T8 obviously had way more specificity to what separated the party and when, but improper positioning led to similar deadly situations in both. Ramuh a lot less severe, obviously.
Thornmarch Ex - There is no raid analogue for this. It's a completely unique fight, where you control your DPS such that you get all Moogles as low as possible before killing any, and then kill a few each phase to lower his HP since he is untouchable until the third killing of the moogles.
Ravana Ex - Arm of the Creator Savage(A11) - Both involve the dash to mechanic. It was introduced in Ravana, but then refined in A11s(and added to). Both have destructible arena portions, though obviously A11s is far more demanding. Both have cleaves, Ravana's is deadlier for comparable at release gear levels. Ravana's is also unavoidable, but it may be shared. Both have skippable adds phase, where the adds phase charges the ultimate.
Thordan Ex - Arm of the Father Savage(A3S) - It was literally their intent to make a very hard extreme fight to improve player skill, and they added in as many phases as possible to it, in order to emulate the sheer density of A3S. Other than the sheer density of mechanics that happen one after the other, they're really nothing alike.
Nidhogg Ex - Alphascape 2.0 (A10s) - Hey hey people, looks like A10s was pretty much a copy paste from what was hard about Nidhogg Ex, except obviously, terrible Midgardsormr fight had higher DPS requirements and more mechanics.
Sephirot Ex - Deltascape 4.0 Savage (O4s) - Both start with a standard boss with its own unique 3~4 minute fight rotation. Both end with a giant omnidirectional boss that has color coordinated side mechanics, with fight specific requirements to solve the color mechanics. Neither boss in its omni-giant form does auto attacks. Neo Exdeath is obviously much more involved, but they're together in my brain pan.
Susano Ex - Second Coil of Bahamut Turn 2 (T7) - Uhh, Not really a lot in common, but the final phase with the sinking border and the lightning stun... kinda like Shriek, where everybody on the wrong side/angle of the renaud got petrified. Obviously that's just one thing, but there aren't going to be good analogues for every Extreme - Savage comparison. That's not really how they design fights heh.
Shinryu Ex - Deltascape 3.0 Savage(O3s) - Somewhat akin to the THordan/A3S comparison. Both are mechanically dense and have similar arena composition, but beyond that have few commonalities mechanically.
Memoria Misera Ex - Sigmascape 3.0 (O7s) - The stacking for the Atlius or whatever is comparable to Ultros phase tentacle stacking. The fire one = Ink mixed with the proximity stun aoe. Similar death walls, though the Savage one is less punishing. More adds and mechanics in O7s obviously, with a much more stringest DPS check. The branching mechanics thing is also similar between the two.
I'd rank pretty much any outlier fight, that's either a unique Ex, like Great Hunt, or a unique normal mode, Battle in the Big Keep, as closer to MSQ by default. Also, keep in mind that the list are just my opinion, that I don't think they == a normal mode or raid fight, just that they're on that side of the aisle in terms of difficulty.
Titan EX
Shinryu EX
Hades EX
Warrior of Light EX
These are the only fights on release that I thought were Raid Tier fights. I was gone during Heavensward but I hear Thordan was also Raid tier.