We just had a big thread on this a while ago, which you can read about here: [url]https://forum.atlas-games.com/t/what-spells-should-every-covenant-have-arcane-connection/7833/1]. If you skip to the end, you can find a nice in-game grimoire of what one of the posters felt were all the "necessary" spells.
To that list, I'd add the following:
Evil Eyeball (Rego Vim) - the Sight-based version of Arcane Tunnel. This lets you cast touch-based effects at Sight range.
Maintain the Demanding Spell (ReVi) - this spell maintains Concentration-based spells for you.
I would agree with the previous poster about some sort of InVi spell to detect magic (I think the touch-based one is either lvl 10 or lvl 15), and the PeVi spells to dispel magic. (The level 10, touch-based spell for dispelling all magic is actually pretty good. PeVi in general grants a lot of free magnitudes to its effects).
For an attack spell, I would recommend the CrAu Call Lightning effect - under optimum conditions (you're at touch range with a stormcloud), it's a pretty low-level effect. Evil Eyeball, listed above, actually grants Sight range to that effect, and Summon StormCloud is similarly easy. Which actually requires 3 separate castings (Evil Eyeball to grant range, Summon stormcloud, then call lightning), so that might be a bit complex. But, if you've got time to set it up, it's a nice "KABLAM!" effect.
Without that, I'd recommend Burning Hands - a touch-based fire effect. Again, using Evil Eyeball, you can add sight range to it, which extends its utility nicely. (But again, that would require 2 castings).