As a 13+ year career DRK .. this is what I think of all REMAs. I haven't played with some of them, but I've heard/talked about the strengths/weaknesses of each many times. I own Liberator and Ragnarok Afterglows, and will eventually get Apoc when I stop being lazy.
Ragnarok - Is probably the first weapon you should make for DRK. It's cheap and powerful. The accuracy +75 (w/ aftermath) cannot be underestimated. Paired with Resolution and lots of buffs, you'll be tearing through most content. But it's not all about Reso! My Torcleavers are just as, if not stronger than my Reso's .. even more so if it skillchains! You really should have sets for Reso & Torc. The accuracy, low delay, aftermath, and powerful weapon skills make it the most popular choice.
Apocalypse - With the Drain 3 Boost (which often leaves you sitting between 3-10k HP), Apocalypse becomes more useful. I wouldn't use it if you have an epic WHM following you around, but if your WHM is dead/disconnected/inexperienced/not present, bringing out this bad boy is the way to go. Catastrophe will keep you healed, while Cross Reaper/Entropy (recovers MP) will give extra damage. If you don't have trusts/no healers, I'd lean towards /RDM subjob .. because it will make you extremely hard to kill.
Not only that, but the aftermath was changed to give you 10% job ability haste. This opens up more hybrid damage taken- & DD sets, meaning you'll TP well, while staying capped in damage taken equipment. Having the ability to heal yourself, will make you less fearful of popping Souleater + Blood Weapon. Apoc opens up a lot of tanking potential .. but it's not a powerful weapon after the first minute (or 2) of a fight. It's a tanking weapon .. so you shouldn't get it first if you can help it.
Liberator - What some players who don't own this weapon seem to underestimate .. is how powerful Insurgency is. Lib boosts Insurgency damage considerably (30%). It's almost level with Reso in most content, and with AM3 up (Occasionally Attacks Twice or Thrice) it's easy to TP overflow. Never use Insurgency at 1000, Quietus (skillchains with itself for Darkness), Cross Reaper, or Entropy would be better options. I usually stick to Insurgency (Light) or Quietus (Darkness). It's easier to skillchain with yourself using Lib than any other weapon DRK has access to; provided you're getting minimal buffs.
I tend to favour scythe to open up skillchain options, or if my party isn't getting an "all you can eat buffet" of buffs. The AM3 is more useful when fighting NMs/mobs who regularly enfeeble with Slow, Amnesia, Paralyze, and other effects which'd slow a Great Sword DRK til there's little point being engaged. Not only that, but this is the best weapon to cast your spells with .. there is no competition. Is it worth doing the Mythic line for it? Absolutely! But only if you love DRK more than getting a Yagrush or Burtgang.
Caladbolg - Is the strongest weapon DRK has. However, there's a couple of drawbacks.
1) It's more expensive than Rag, and you got to collect all the abyssea items as well. This is not for someone starting out, it's for someone who has most of the best equipment available.
2) It doesn't have extra accuracy like Rag. It shouldn't matter though .. depending on the content you're doing + the buffs you're getting + the amount of accuracy you have in equipment + food + any other outside sources of accuracy. Rag wins when you need the extra 75 accuracy.
It's easy to get enough accuracy for any content outside T4 Reisenjimas and possibly some parts of new Dynamis (so 95% of content in the game). The Torcleavers this beast can dish out is terrifying. You should be getting close to Mighty Strikes Rag Warriors with this (or beating them), as long as your Torcleaver set is top-notch. I'd love to get one .. but I know it would make my Ragnarok obsolete on most things. It's not a priority at the moment. Hopefully the strength of this weapon continues to increase with time.
Redemption - I've got one of these on 90, and when I played with it in the Abyssea era I enjoyed it. It was my first Ultimate Weapon, but I can see why it sits at the bottom of the list. It doesn't have much in terms of accuracy, or other special abilities, but what it does have is amazing "white" damage. For those who don't know, white damage is typical melee hits. It has an aftermath which doubles/triples damage.
I'd say that .. if you fought a NM where you could only WS for the aftermath (and then nothing else due to a mechanic .. but this is very situational), Redemption would probably come out on top. I love the design and WS attached to it, but sadly neither it .. or the WS have enough to bring it close to the great swords. And it's not worth the extra money, ya better off saving it for a Rag.
Anguta - I want this but I'm getting the dagger for my first Aeonic .. and probably Fomalhaut as my second, so I won't get this bad boy for a while. But it's easy to find out what people think about it .. it's a glorified skillchain machine. You can 4-step (and more), and it will always beat a Liberator if you're geared/have enough buffs to do it consistently; which is most of the time.
The Ultimate skillchains make this the best scythe in the game. And I do love my Liberator .. I really do, but I can see why any DRK would prefer DDing with this. However, I don't know if it's much of a team player. It's hard to say how it stacks up when you have other DDs wanting to WS in to account. If you can skillchain with another Aeonic DD, I reckon it would be great .. otherwise you might want to switch to Calad or Rag.
If you're soloing Apex, Anguta makes them hella squishy (there are several videos of DRK's showing its potential on these mobs). If you're fighting anything higher than that .. you're having to decide if using Cross Reaper/Entropy/Quietus is better than Reso/Torc spam. As always .. it comes down to what you're doing, and who you're doing it with.
Conclusion - So outside of Redemption, every DRK REMA has its uses. Rag if you're new. Apoc if you love to tank. Calad if you want perfect parsing potential (make them WARs cry!). Lib if you love Insurgency, OAT, and casting magics. Anguta for pure skillchain power. And Redemption I guess .. to say you got it! And that's all I can think of. Feel free to add your own experiences.