All decent to really good. Sheoldred is very oppressive (to your opponents ... and isn't that the point
) In my playgroup, Sheoldred always draws the hate. I have personally stopped running her so frequently in my decks. I personally didn't like
Dark Deal for my discard deck. I just wanted to straight-up make my opponents discard; I didn't want them to be able to draw replacements. (I'm evil like that.) I also personally don't really like
Bontu's Last Reckoning, but that is mainly because I have a
Damnation,
Black Sun's Zenith, and many other black board wipes. Gonti is awesome. He can go into just about any deck. Casting your opponent's spells is always fun. I had a deck with Gonti as the commander. It's in my "Retired decks" folder.
Once you get past having appropriate land counts and non-land mana sources for your deck, the individual card choices come down to personal choice, the cards that you have, and the cards that you can obtain. Ob Nixilis is a decent commander choice for a mono-black "good stuff" deck.
Happy Brewing