"Free Organ Accompaniments to One Hundred Well-Known Hymn Tunes" by T. Tertius Noble is solid, in my opinion. I also sometimes use them for simple postludes on the recessional.
The obvious (and my primary go-to) are the collections by Noel Rawsthorne (now you can get a mega book with 400. His are great because they are very playable, (if tame by comparison to some, but safe in nearly every context) and ultimately rather approachable. They add flair without going off the deep end.
I love the T.T. Noble collections. They really are splendid. Oddly enough, I haven't gotten the Noel Rawsthorne yet! But I do have a book by Gerre Hancock that I adore!
I make occasional use of Colin Mawby's "Hymns for Occasions". Some arrangements are more to my taste than others. Haven't tried many other collections.
I wrote a delightfully adventurous descant & reharm for the final verse of "Holy God, We Praise Thy Name," which I'd be happy to send to anyone interested. PM me with your name & email address.
I have user all of the collections mentioned above frequently. I also use Hal Hopson's Organ in Worship compilation. The reharmonizations almost always retain melody, so they are very friendly to singers (and busy organists.) I think using a variety of the various collections is best as most of composers have rather similar reharmonization formulations. All that said, Noble's collections are the best!
I humbly add my collection - during COVID I created an alternate harmonization to every hymn in the St. Michael Hymnal - some 250. The entire set is here:
However, you can purchase individual harmonizations at a very low price - just search under Gregory Hamilton and the hymn tune. Sometimes there are more than one harmonization.
There's a reharmonization/key change of HYFRYDOL in Festive Hymn Tune Reharmonizations by Jason D. Payne that I've become quite fond of. I can recommend his work for easy to moderate difficulty works that sound quite appealing to the average layman.
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