I remembered a few more Commander gems.
Rishkar's Expertise: Draw lots of cards, and then cast one for free. Absolutely amazing.
Notion Thief: It has
flash, so it really catches people by surprise.
Mental Misstep: every one mana spell in commander is worth countering.
Sol Ring,
Mana Vault,
Elvish Mystic,
Serra Ascendant, all of them.
Siren Stormtamer: This card is a new favorite of
Azami, Lady of Scrolls and
Edric, Spymaster of Trest. See also:
Judge's Familiar.
Gideon of the Trials: His emblem can be activated immediately, so he prevents game winning combos right away.
Call to the Kindred: tribal decks have always been popular, and this card fits right in.
Sudden Death,
Sudden Spoiling,
Sudden Shock,
Trickbind: I don't think people really understand how important split second is (most green decks use
Krosan Grip, so that's good). You are stopping someone's combo, and they can't do anything in response.
Tragic Arrogance: you get to chose what stays and what goes. You leave your opponents with the worst stuff (like a goblin token) and you get to keep your best stuff.
Goblin Sharpshooter: This guy tears through the board. Deal one damage to
Elvish Mystic, it dies, Sharpshooter untaps, so you can do it again. You can destroy anything with one toughness at no consequence.
Defense of the Heart: This card very obviously wins games, so why is it in so few decks?
Chancellor of the Annex: there was a full cycle of Chancellors, and this is the only good one.
Spreading Plague: this will mess the whole board up, and force people to really think about what they're going to play.