For non-classical Releases:
Is the current community standard to source Release title from spine (if present) or cover (if spine not present)?
Do others agree that formalising this by including it in the Style Guideline would improve editing efficiency and Editor Experience (EX)?
See an example where clarity and documentation around this would enhance editing at https://musicbrainz.org/edit/54821602
Usually the spine seems better to me because it is most of the time on one line, so that matches our release title field more conveniently.
BUT Spines are small and sometimes the title is cut off to fit in that limited area. So the full title is in these cases on the cover or CD label.
Overall IMO, it’s better to make a mix between spine, front cover, CD label and booklet (in that order of preference when they conflict).
Sometimes even this guideline to which I agree, would give results I don’t really agree with…
It’s why I don’t know if it’s really better to have an official guideline.
CATCH A FIRE by THE WAILERS for instance.
More recent editions, show BOB MARLEY & THE WAILERS:
So it’s not only a matter of order of priority…
Sorry for my puzzling post…
In case of conflicts (only) between package elements, there could be an order of preference, but the priorities are trying to find the essence of the data:
And it’s more easy to list that than to declare that this part of package is more important than the other.
Actually the main benefit of spines for me is that they will most of the time choose for me if the second part of the title is a subtitle separated with colons or if it is to set between brackets or other separators.
I don’t think that the front cover should be the primary source (or even is a good source at all).
There are thousands of cases where either the band name or the album title or both are printed on a front cover in a way that we do not want to replicate in MB.
Some examples:
Depechemode (with different fonts for the 2 words): https://www.discogs.com/release/7052742
DEPECHE MOdE (with larger D and M): https://www.discogs.com/release/10472909
DEPECHEMODE (with slightly larger font for MODE): https://www.discogs.com/release/9962043
dEpEChE MODE: https://www.discogs.com/release/42693
DEPECHE MODE (with differently stylized D and M): https://www.discogs.com/release/2401723 . https://www.discogs.com/release/4669922
All of these are credited to Depeche Mode.
In many cases, artist or title are not printed on the cover at all.
Even though this is not mentioned in any guideline, common best practice was to use spines, media labels etc. as references for titles of physical releases because they (typically) do not have any graphical overhead.
Since these sources are missing on digital releases I think the best source for printable artist names and release titles are the official artist or label websites or social media platforms.
Alternatively, we could fall back to the titles of corresponding physical releases.
BTW, I wouldn’t use titles from big sources such as Amazon, Spotify and iTunes because they might decide according to marketing needs (e.g. Prince instead of Symbol) and are not always consistent among each other.
Surely the over riding priority here is Artist Intent? What did they originally want? What is in their discography on their own website? What do they call the album when talking in interviews. It is their work. 
I have added CDs to the database where the spine clearly had a typing mistake.
I have seen early album releases with typing errors. Later releases with those errors corrected.
We have UK albums released in the US with clearly confused typos added by the USAians attempting to work out what “colour” is.
Compilation albums should always be treated as dubious as there we have some company just trying to make some cash by slinging a heap of tracks together.
Same with Amazon \ iTunes \ Spotify - they don’t really care about accuracy as they are a shop.
The Artist on his\her own site, YouTube, Facebook, Twitter, etc is what I treat as key.
Personally if I am looking at a CD in my hand I am more likely to trust the REAR cover. Followed by the booklet. If I see the title written in different ways on that same release, then I will use the most common version. So if booklet and cover are the same - I’ll use that.
It will be almost impossible to make a single rule as this will lead to arguments when someone wants to pedantically follow the rules and add in a title that is clearly a typing mistake.