Greel, Mind Raker Greel, Mind Raker strikes me as the worst horror commander because its discard ability targets a single player, and thats ineffective in EDH
1st Place: Manifold Mouse Ability versatility, nice, balanced cost, plus a chance to double up the card itself for the exact same cost when the opportunity presents itself
Nettlecyst does the same but tracks all artifact and enchantments, so its even more explosive in decks that go wide those card types
"Shield" is not an ability that creatures have and shield counters are not keyword counters
Once you've got to grips with your favourite deck, have a read of our guide on how to build a commander deck to find some tips on building and upgrading
Aurelia is a bit pricey at six mana, 3/4 with flying, vigilance and haste