The 2018 CDs use the identical mastering used in the 2015 Onkyo and iTunes releases, so if you already have those, don't buy the new CDs. They are a definite improvement over the '98s, as they eliminate the clipping and distortion that made the '98s unpleasant and fatiguing. They are NOT an improvement over the original CDs or the vinyl reissues from 2014, IMO. They are too loud and compressed, so that the the peaks (particularly the percussion) lose their oomph. The bass is boosted in the new remasters though, so some may prefer that. Because the new remasters are louder, one may think they sound better -- that is the fallacy of the loudness wars -- until you compare them to the originals with the volume cranked up. There should be added clarity in the new remasters, due to the use of a better source (the original 2-track master tapes), but the compression muddies things unfortunately.
The best sounding (#1 ranked) releases of all are, IMO, the 2014 180-gram vinyl LPs.* The 2014 LPs are much, much better mastered than the 2015/18 digital product. Like the 2015/18s, they were mastered using the original 2-track master tapes. But they are not nearly as compressed as their digital counterparts, and thus sound worlds better -- too bad, because a well-mastered CD or hi-res digital product using that original source tape SHOULD have been the best version. The LPs have improved clarity and separation, and they are much more dynamic, packing a real wallop when turned up loud. They are the go-to, archive-quality versions of these albums. If you own a decent turntable, then run, don't walk, to buy those before they go out of stock. (Note: I had heard that the UK releases of the LPs used a better pressing plant than the US releases, so I bought the UKs. I cannot speak for the US versions, but the UKs are well pressed with little to no surface noise, and they sound fantastic.)
I rank the original CDs a close #2 (and the best digital versions).* They are very dynamic and have a rich, warm, natural sound, though the bass on Piece of Mind seems a little thin. The 1995 Castle CDs used, I believe, the same digital masters as the originals, so they are tied at #2. I'll rank the original LPs #3, though my copies are so heavily played and not very well maintained (hey, I was a teenager) that they have a lot of surface noise, which makes it hard for me to fairly judge. A cleaner copy could be as high as #1. But don't spend too much time or money hoping to find a near-mint version of the old LPs -- just get the 2014s and you'll be happy. The 2015/18 digital versions are ranked #4. And the '98s are a distant #5 -- they are hideous garbage and should all be destroyed. So, if all you currently have are the '98s, the new remasters are a significant improvement. But you are much better off getting the 2014 LPs (new) or the original CDs (used), though they are going to be a bit more expensive.
* Footnotes: [1] I actually prefer the 198x U.S. Capitol CD over the 2014 LP for
Number of the Beast. [2] To be precise, there were multiple masterings of certain original 1980s CD releases. While they are not identical, they all sound pretty close for the most part. There is a whole thread on this stuff in the Steve Hoffman forums if you really care. I think you're fine with whatever you can find from back then, but I do recommend the U.S. release of the self-titled debut over the European or Japanese releases, as it sounds better and has "Sanctuary." Don't spend a ton on the Japanese "Black Triangles" other than as a collector's item, as they sound pretty much (or exactly) the same as the US versions.
Happy Thanksgiving,
CFH
P.S.
@Forostar : I cannot find your poll in which you asked people to vote whether the new remasters are the best versions. But my vote would be "no" in the poll for everything EXCEPT
Brave New World and
Dance of Death -- the 2015s sound better on those.