for me, most guitars sound and play best in Eb, with any good pickup
I like the extra growl of drop C or C# standard, but most guitars lose a lot of everything with that
but for a true doom sludgy tone you need that growl and the mudded up sound, so I would only bother with string tension and tuning stability
if you use low C that's too thick and tight, you end up losing some sustain and the bass gets too woofy
if it's not thick enough, then the tuning oscillates too much everytime you pick
12-56 should be fine for C standard