There are a lot of Virtues and Flaws in ArM5. Pointing a new player at the whole lot is likely to lead to choice paralysis.
So, where would you point someone? These are not the powerful choices, necessarily, just the standard ones that fit well with common concepts. I'll start.
Affinity and Puissant are good for any character type if you want to build a specialist. For magi, add Magical Focus and Potent Magic.
All characters should take a Personality Flaw, and it is a good idea for it to be Major if your character is a companion or magus. Similarly, a Story Flaw is a good choice for a magus or companion. These are good "value for money" and give the troupe a clear handle on what sort of stories you want your character to be involved in. If you aren't sure, it is a good idea to decide now, because in Ars Magica you will often need to go looking for the story.
Fighting characters need a Virtue that lets them learn Martial Abilities, like Custos, Knight, or Warrior.
Then there are the Virtues that are a good place to anchor a concept. Elemental Magic for a magus, for example, or Landed Noble for a companion.
What else might people suggest? Any ArM5 book is fair game.
I wouldn't generally recommend all magi and companions have a major personality flaw, though - if a character is going to have an extreme enough personality that it regularly causes them trouble (which I understand is the intent of a major flaw), then that should be something a player would want to think through before committing to. (edit: I don't think I've ever made a character with a major personality flaw on that basis.)
Social status, personality flaws, and story flaw should always be selected (with the obvious exceptions for grogs), and they should always make a statement about the character concept and the desired stories. I would not point to specific virtues and flaws; it is rather a matter of scanning each list and picking the one most appropriate.
Magi should normally have a major Hermetic virtue. There are relatively few to choose from, and each one gives a focus area which may help channel other priorities. Again this is a «scan the list and pick one» exercise.
Then there are the virtues to open up martial, arcane, or academic abilities. Only if the concept dictates should this be considered, but it must be addressed because they cannot afford to forget.
I think I would recommend skilled parens to magi, but I must admit that the main reason is that the magus would otherwise feel underpowered compared to 3ed ... but then, a first character has to work well in the first couple of stories; one can always start over with more experience before the players enters the long game.
At this point I am stuck. In fact, I think a list of suggestions per social status would be in order. Such lists already exists for Hermetic houses, but they are possibly too long.
I's start with an idea, like "I want to play a magus who is mute". This idea can be a single virtue or flaw, but doesn't have to be. For mages, it is often either a focus or a major virtue.
That gives you the mute flaw.
From there, most virtues and flaws fall into place by themselves:
I'll need a virtue (two actually) to cast spells without speaking.
I need to be able to communicate without speaking, so I'll need virtues that boost imaginem or mentem (or whatever means of speaking is used)
What does mute have to do with the personality and the story? Is it a curse? Story flaw and personality flaws found
In the end, I usually have two or three slots for virtues and flaws left, which I fill somehow (skilld parens for easy xp, some social virtues if the person is social, improved characteristics often makes sense).
This way I get characters who have about 2/3 of their virtues and flaws grouped around a concept, and then some leftovers to prevent the character from becoming too onedimensional.
So what we require is not the easy rulebook for newbies that @David_Chart appears to be writing, but an advanced character generation software package with machine learning features, so that you can type in exactly what you suggest and get a list of relevant virtues and flaws for your concept.
When I create a character, I start by picking a social status, one story flaw, one personality flaw and one learning virtue then I stop and I consider options arround the concept I have in mind. There's usually a Puissant something somewhere and a Focus is always nice to have.
But a new player who doesn't know ars magicka at all? Funnily enough, the one thing I don't do during the first game session is giving them the book. Ars has a steep learning curve, and the last thing I expect a player to understand is how spontaneous, ceremonial spontaneous, formulaic and ritual magic work after game 1, let alone what virtues and flaws might be useful.
This was how I did my first game:
The second time we introduced a player in a saga, I was not the gamemaster that time, but again, instead of giving her a full fledged magi to build, what we did was introduce her as an apprentice and she learned about the game as I opened her arts and taught her how to create her first spells.
I don't have a lot of experience outside of magus creation.
When playing with new player I try to advice him to pick flaws based on the kind of story he wants to get into (story and personality flaws are good for this) and virtues based on what he wants his magus to be like.
I also try to make him pick his house first and give him a broad Idea of archetypal magi of that House, using archetypes of the basic rulesbook, the founders/Major figures of the House and/or past magus we played as a troupe in previous sagas as exemples. In this process I try to point out generic virtues that fit any/most magus (xp virtues comes to mind, as well as magical focus) and those whose "values" are very dependent on the concept (HoH:MC Merenita virtues/flaws are a very good exemple of this as Tugdual pointed).
Thinking about It, it feels like xp virtues are more or less mandatory if you're playing just out of gauntlet.
I think it's more a newby tells a Story Guide their idea and the Story Guide can point them in the right direction. Putting that concept in the Ars Majica 20th anniversary edition as a side bar might be a good thing.
I think turning people away from certain virtues and flaws is important. A symbol to say recommended for experienced players. In that group would go Diedne magic & Mercurian Magic.
That's an excellent idea for those who are so lucky as to start with an experienced SG.
Some groups are not so lucky, and to make a 20th anniversary edition worthwhile, I think the publishers have to make it easier for them to take up the game.