At the risk of stating the obvious, it very much depends on the vocal range of the singer, and the range of the song. For myself, I often find F to be a good singing key, and it can be played fairly easily on a C/G anglo especially if you are mostly playing chords and not too much melody.
I have a baritone in F/C which is nice for singing with.
I appreciate that one can play in various keys on eg C/G but what home keys do people prefer on their concertinas. It would be nice to have an array before me but I haven't If anyone can direct me to any CDs or YouTube clips I'd be grateful
I know Steve Turner told us in a workshop he likes a lower timbre. Brian Peters and ( I think ) John Kirkpatrick use an Anglo Tenor /Treble for accompaniment and I know people who play fiddle and sing to that, equvalent to the RHS of the Anglo
I suppose I'll just have to mess around with a keyboard to see what suits the keys I like singing in. (F, C, Bb, Eb, Dm)
I commented elsewhere that I find it very hard to join in with hymns f I go to funerals or weddings in church but not at carols in our South Yorkshire pub sings (piano or electric organ or best unaccompanied). I wonder if the church kicked out the village bands and their keys when they put in organs, or was it choristers that replaced the congregations chosen keys?