If this were a list of cards I wished I had more physical copies of, it would be Teleport and Force Push. But I rarely forget to include them (or a similar substitute if I don't want to proxy and don't want to pull copies from other spell books).
I tend to forget to include healing... or at least opt not to in favour of other spells, then wish I had at least a couple during a given match.
And I tend to forget a reasonably high-damage quick attack spell to use as a finisher when my opponent is down to <6 health and I need to finish him/her in the opening quick-cast phase before he/she can heal (or Teleport away or put up a Reverse Attack/Block, etc) or before he/she beats me to the punch and finishes my mage first. Examples: Hurl Boulder, Fireball, Force Hammer, Luminous Blast, Lightning Bolt, etc. It's usually my Necromancer or Forcemaster or Priestess/Priest or Anvil Throne Warlord or bow-based Beastmaster who forgets to pack such a spell. I'll have at least one of Acid Ball and/or Surging Waves and/or Flameblast and other low-damage utility attack spells, maybe one or two zone attacks, and I might even have a full-action finisher like Hurl Meteor... but forget the importance of at least one quick-cast finisher.