Alpine Moon Blood Moon - These are for decks that rely heavily on their manabase to access colors on time, like Esper/Jeskai Control, Tron, or Sultai/Jund/Abzan midrange. For decks that make use just a single special land, like
Inkmoth Nexus or
Spire of Industry can have just the
Alpine Moon come in.
Dismember - This is for times when you just need to kill something big real fast, like a Gurmag Angler, Lyra Dawnbringer, or Endbringer.
Gaea's Revenge and Thrun, the Last Troll - This gives me another option to fight against control.
Veil of Summer - This is a reactive card, and very good against blue and black control decks.
Fracturing Gust Collector Ouphe and Reclamation Sage - Artifact decks are plentiful, so these hit them hard.
Pyroclasm Kozilek's Return - This is good against any decks playing lots of small creatures like Elves, Soul Sisters, Affinity, Humans, Spirits, or burn. Takes out your own mana dorks, but they make poor blockers anyways, so these sweepers are worth it.
Ruric Thar, the Unbowed - This is for any spell-heavy opponent like combo, control, Phoenix or burn.
Surgical Extraction - Used less commonly these days, I may bring this in against decks that have specific cards that can hose me, or against control so I can eliminate a board-wipe or planeswalker after dealing with it.
Walking Ballista - This is great against decks that run small but important creatures like Druid Combo, Infect, Humans or Burn. Also machine-guns mana dorks like Hierarch, Birds or Elves.
Relic of Progenitus Grafdigger's Cage Scavenging Ooze- Graveyard decks are not prevalent in my shop so I don't have much need to fight it, but for larger metas, Relic and Cage are good, quick graveyard tools.