I don't think the x-ref is helpful.

@@ -12 +11,0 @@
-<xref type="see" seq="1253840">傑作・1</xref>

GG5: a work of the highest quality; a masterpiece; a masterwork; one's magnum opus.

Having a solitary plural gloss sitting in the middle of singular ones looks very odd, Anyway, I doubt it's actually needed; the other five are quite sufficient.
Also, we are fairly wary of quoting verbatim from Eijiro - see the Editorial Policy.

@@ -15 +14,0 @@
-<gloss>greatest works</gloss>

I do see that we don't usually do (s).  This situation seems exceptional to me, I just don't know how to address it.

Suggestion #2: just make "greatest works" plural (also a deviation from our style, but it is sometimes done in mostly-plural cases).  Eijiro even does this:
https://eow.alc.co.jp/search?q=最高傑作
crowning work
greatest works
magnum opus

The issue here remains that "highest", "crowning", "greatest", "magnum opus" all strongly imply that this is singular. (and masterpiece and masterwork, while not strictly defined as singular, are quite compatible with this interpretation). 　最高 also seems like it could be "the absolute best". 

That makes the whole entry misleading. If you were seeing this in a J->E context, you could easily be lead to believe "there must be only one thing", when 最高傑作 does not have this limitation, and is quite often plural/uncountable.

I assumed it was the #1 best thing, until I went searching for examples and discovered it was not.  That's what I wanted to address for other dictionary users.

@@ -15 +15 @@
-<gloss>greatest work</gloss>
+<gloss>greatest works</gloss>

We don't usually do "(s)" endings.

@@ -16 +16 @@
-<gloss>greatest work(s)</gloss>
+<gloss>greatest work</gloss>

One of...
傑作の一つ	14858
最高傑作の一つ	6851

https://tr-ex.me/翻訳/日本語-英語/傑作
最高傑作は
-masterpiece
the perfect album
the best works
finest work

https://eow.alc.co.jp/search?q=最高傑作
greatest works
(among others)

Google returns some nice examples when simply searching
最高傑作 "greatest masterpieces"

I would personally do this:
(greatest) masterpiece(s)
But I feel like this will be too ugly to be accepted.

xref in an attempt to make it clear that you might be saying more than you mean to with "最高傑作". "傑作" is already legitimately "a masterpiece".  This entry is "among their greatest masterpieces", or something like that.

The selection of glosses here, along with 最高, give the strong impression that only one work applies.  But ngrams for "最高傑作の一つ" give away that this is not necessarily the case.  

In the structure "one of their "..., none if the existing glosses seemed to me to apply.  "One of their finest works" seems most natural to me, "one of their best/greatest works", one of their greatest masterpieces... 

"best" (or "greatest") seems more appropriate here than it did in "傑作"  If you search "傑作" and "best work", the results will tend toward "最高傑作". 

masterwork can legitimately go with 傑作 alone. That said, "masterpiece" seems like the most common gloss for 傑作 or for  最高傑作, so I've moved it up.

@@ -11,0 +12,3 @@
+<xref type="see" seq="1253840">傑作・1</xref>
+<xref type="see" seq="1253840">傑作・1</xref>
+<gloss>masterpiece</gloss>
@@ -13 +16 @@
-<gloss>masterpiece</gloss>
+<gloss>greatest work(s)</gloss>