Some spells I consider extremely useful to any covenant. Some of these spells need only one magi to have a copy. Some spells need copies in the hands of every magi.
A spell to degrade arcane connections. Doing it one step at a time is good enough. But without that you can't write your own letters, enchant and share items or so much else.
A spell to detect vis. Not necessarily level 10.
Demon's Eternal Oblivion (probably lower than level 10). Cast on everything.
A room wide demon's eternal oblivion. (possibly lower than level 10.) Cast on everything.
Perdo Vim for dispelling infernal effects. You may not be able to see through demonic illusions but you can blast them to pieces. BTW, cast on everything.
A spell to transfer vis. This is going to be level 15.
A spell to create images or sounds with sufficient clarity to be used to communicate at arcane connection range. You may want a duration, but it can be done at just level 10. Do you have any idea how many problems can be solved with cell phones?
A sight range attack spell. A flight spell. These two handle any threat that mundanes can muster if your attack spell doesn't completely suck. Assuming you have time for lift off at least.
A spell to create 10,000 cubic paces of dirt for a diam. Because nothing says: "I will bury you", like burying them. This is more or less the poor man's city destroying spell. You'll never know when you need to flatten a random city.
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).
Assuming that you are making the magus up in conjunction with a saga, then it is good to check with the other players what they are doing. For example, Wizard's Communion is great if everyone knows it, but sod all use if only one magus knows it.
And also, if possible, think about the saga, and what you think your character will be doing. For my money, the most important thing is to take spells that will be useful in your saga. Lots of spells that will be cast every session in one saga, will hardly ever be cast at all in another saga.
The battery of 3 anti-demon spells of Lamech would be fairly useless in 90% of my sagas, basically because we do not like demons much and they have a limited presence.
I would think of the spells in thematic usefulness, and then go hunting for specific spells:
Attack spell (does not need to be very big)
Defence spell (a wall is always useful, bu generally higher level. A pile of mud or similar can also work here. Invisibility doubles as a defence and utility spell, and rise of the feathery body as well)
Recovery spells (bind wound, recovery bonus)
Economy spell (something that can give you money or savings, like one to chop wood easily, create fires without fuel...)
Comfort spells (cloak of duck feathers, warmth spell)
You may want to check your non-fatiguing spont levels: there are a number of decent lvl 3-5 effects that you might be able to pull off without actually having to memorize anything. One of the other benefits of ReVi Arcane tunnel spells is that they work with spont magic - therefore, as long as you can get your spont magic to Touch level, you can cast them at sight or Arcane connection range. Similarly, Maintain the Demanding Spell works with spont magic as well - so if you can get your spont spells up to Concentration, you can get them up to Moon duration. So while there may not be a number of combat spells you can cast with these combos, there are a number of strategic or non/combat effects that you can build.
Similarly, check your spont levels for MuVi - while you can't use metamagic on spont magic, your metamagic itself CAN be sponted.
With this in mind, talk with your troupe about what the MuVi guidelines actually mean. (It's one of those things that you need to work out amongst yourselves.) Specifically, what does the "minor change" mean? If it means "keep the T, D, and R, and general guideline, but change the application of the guideline", then your sponted MuVi spells are REALLY FREAKING USEFUL - especially for Mentem effects, IMO. Check to see if "minor" includes "change realm" as well. (The rules themselves don't specify.)
MuAq(Au) 20 - so that either saves you one or sets you back one.
There are some excellent suggestions above, but I'd just like to add a few suggestions for InVi spells.
Personally I've grown rather fond on level 10 spells for recognizing/finding Vis and aurae.
You'll have to design your own for the moment, but perhaps someone will get around to publishing them some day.
Maybe it's because the 'find vis' spell is so blindingly easy (Base 1) that it was previosuly glossed over, believing it to be firmly inside Spontaneous territory? Anyway, that's no excuse since you want to add magnitudes for a good duration and a useful Sense in order to make it viable for vis searcning. Also an In 0 Vi 0 magus can still invent (from text) and cast it easily enough, while hard pressed to Spont it.
But oddly enough this has not been a thought process for any of the published magi.
