Wizard's Leap and Wizard's Sidestep are extremely useful if you run into trouble.
Bind Wound can be a literal lifesaver.
Disguise of the New Visage or Disguise of the Transformed Image can be very useful when you don't want to be recognized
ReVi spells to support your spontaneous magic.
In general, MuVi can't support spontaneous magic, but ReVi can.
Of particular usefulness:
A D:Ring, Sun or Moon version of Maintaining the Demanding spell. Then, you can cast a spontaneous spell with D:Conc, and "pin" it in place. This means saving a magnitude or two with spontaneous magics that need longish durations. Which may not seem much but it actually is - particularly with botch-and-fatigue-free non-fatiguing spontaneous magic.
A R:Sight version of Wizard's Tunnel. At the cost of 1 extra round, and the risk of an opponent using the tunnel to get back to you, you can extend the range of your magic from Touch to Sight.
10 Clothe the Naked Form (MoH 58) for disguise or after shapechange but need Finesse roll
10 Words of the Unbroken Silence (ArM5 148) only really shine if you can cast it without gestures and words
10 Lamp Without Flame (ArM5 140) concentration duration light
05 Palm of Flame (ArM5 139) can igniting fire and give of light.
Call to slumber (ReMe 10) - handy for putting guards to sleep, and also dealing with supernatural threats that sleep - it works on anything with a mind that falls under Mentem, so there's a range of intelligent supernatural beasts that can be put to sleep.
Bind Wounds - if you do not have this, make sure you adventure with at least one (preferably more) grogs that know chirurgy well or another magus who does. Without the ability to bind wounds, the true brutality of Ars Magica injuries can hit your group when things go bad.
I second the love for Wizard's Sidestep, it has saved more than one of my characters from death.
Magical movement (wizard's leap or a low-level flying spell) can help reaching awkward places in adventure.
Some utility spells I like to take if they fit without messing further with arts that haven’t been listed above:
Pass the unyielding portal MuHe 5 Core book
Intuition of the Forest InHe 10 Core Book
Conjure the Sturdy Vine CrHe 5 Core book
Trap of the Entwining Vine CrHe 15 Core book
Probe Nature’s Hidden Lore InHe 4 Core Book
Physician’s Eye InCo 5 or Revealed Flaws of the Mortal Flesh InCo 10 Core book
True Sight of the Air InAu 15, Clear Sight of the Naiad InAq 5
Image of the Beast InAn 5
Circle of Beast Warding ReAn 5
Taste of Spices and Herbs MuIm 5
Notes of a delightful Sound MuIm 10
Words of Unbroken Silence CrMe 10
Sight of Transparent Motive InMe 10 or Perception of the Conflicting Motives InMe 15
Recollection of Memories Not Quite Lived MuMe 4 (Moon duration version is only level 5)
Anyway, that’s probably more exhaustive list than you wanted. Many of these seem good flavor for a magus to have, some literally. And in addition they can often be used in inventive ways depending on circumstances.
I feel like Unseen Porter should be on this list, possibly Hand of the Invisible Thief as well. I've gotten up to a lot of mischief with Unseen Servant in AD&D2e games so I feel like anything that emulates telekinesis (or possibly an invisible ally) can find a lot of potential uses in the moment.