I rarely have any favourites regarding
Tolkien; I rather view all texts as a whole, but of the things I've read in HoMe and UT, which aren't that much:
Everything in UT is completely great.
No need to elaborate further there.
I didn't think that I (the first volume) was so interesting, knowing that it's mostly obsolete material, and the material that isn't obsolete was so similar to the published
Silmarillion. Some things like the traveller
Eriol were interesting though, and that cottage of lost play.
The Fall of Gondolin in II is something to be mentioned though. I haven't read it in full myself, but according to others it's very good and exciting, and it can with some fairly rough modifications be viewed as not abandonded material, because Tolkien never rewrote it, and adds many pages to the legends of the
First Age.
I think
The Lost Road in V is interesting with it's linking of modern times to ancient times, and it would be a good topic for a thread, I think. I know that the idea was later rejected, but the conversations between
Elendil and his son
Herendil can, with some modifications (thinking of Herendil as
Isildur, for example), I believe, even be regarded as not rejected by Tolkien because they don't seem to contradict much with his latest views concerning the Downfall, and the conversations seem to be a quite natural part of the build-up to the Downfall, thus adding more material to the
Second Age, of which there isn't so much (at least in the amount of text) written.
The development of the conceptions of
Númenor until the
Akallabêth are also interesting to follow in V (
The Fall of Númenor) and IX (
The Drowning of Anadûnê).
I've read a little of the beginning of VI and Tolkien's first views of
Bilbo's doings in LR are quite amusing (getting married and all that). I am sure it's very interesting to follow the development of LR in the LR HoMe's, and I know there might be some material that one may take as small additions to the final LR, and that makes them especially interesting. In VII one can find additional material about Tolkien's views of
Lothlórien and it's conception of time, for example.
I've yet to read
The Notion Club Papers in IX but I am sure it's interesting.
I haven't read anything of X yet but I know a bit about it and that it is a must read for every true Tolkien fan.
Athrabeth Finrod ah Andreth and
Myths Transformed are very important texts concerning Tolkien's latest conceptions of the mythology.
I have acquainted myself with the
Wanderings of Húrin in XI, and I know that it can somehow fit in with the ruin of
Doriath and the rest of the legends of the First Age, so it's great that it adds many pages to that Age. In
Quendi and Eldar there is very interesting information about the very first
Elves.
XII is an excellent volume in that it contains a lot of material that can be added to the mythology. Of the things I've read (or know) especially
The History of the Akallabêth,
The Tale of Aragorn and Arwen,
Durin's Folk,
Of Dwarves and Men,
The Shibboleth of Fëanor,
The Problem of Ros,
Last Writings,
Of Lembas,
The New Shadow, and
Tal-Elmar all add more material, in some cases very important.
It will take a long time, but I intend to read the whole HoMe series, and everything else Tolkien ever wrote as well, probably.
I am not just interested in reading everything that can be considered part of the latest mythology - following the development of the texts is also interesting and makes you understand Tolkien's works better.