Note that there have been several threads (e.g. this one) with a similar theme. But to answer the OP’s call for my favourite 5 Virtues and 5 Flaws:
VIRTUES
The Gift. Well, the entire game revolves around this Free Virtue, right?
Animal Ken. Not only a staple of fantasy, but incredibly useful for a resourceful and otherwise minor mundane character: animals are everywhere, have a vast array of talents, and are often quite easy to befriend/bribe. This is even better in the Folk Witch version (HMRE) - remember that any grog can be a Folk Witch by taking a Free Social Virtue.
Merchant Adventurer (C&G, Minor). For a Minor Social Virtue you get a ship, a crew (make sure you design every member of the crew for maximum colour and munchkin-power!), a source of income large enough to support a covenant as well as the entire Labour-Points mini-game centred on it, and lots of story potential. Great in combination with an appropriate Regio Network (RoP:M) or with the Lone Redcap Minor Virtue (HoH:TL).
Mythic Stamina (HoH:TL, Minor). Fantastic for munchkin mages: choose magic as your specialty and you get rid of that pesky last botch die. This immensely empowers fatiguing spontaneous magic, and thus it's great in combination with Virtues such as Life Linked Spontaneous Magic or Diedne Magic that boost it. It's a Heroic Virtue, so you need to enable access to it: the Legacy Story Flaw is a perfect match.
Enchantment + Grant (RoP:F, Major+Minor). This pair of Virtues is worth building an entire Companion around (you'll need a source of Sympathy/Antipathy too). The reason I like it so much is that on the one hand it's immensely versatile mechanically, on the other hand it's strongly constrained thematically by the scope of the character's sympathies/antipathies; for lovers of Nobilis. See e.g. Yvonne for this pair empowered by a Faerie Sympathy in swordfights.
FLAWS
Outlaw leader. For a Minor Flaw your Companion gets access to Martial Abilities, a bunch of followers, a bad Reputation which you can occasionally find good uses for, lots of story potential and a good reason to associate with a Covenant.
Failed Monk/Nun (RoP:D). A staple of fiction (from Robin Hood’s companion Friar Tuck to Imperius in Ladyhawke), it’s perfect for a character with church ties but a strong motivation to associate with a Covenant. It's also neat if you want a grog with access to Academic Abilities but don't want to spend one of your three precious Virtue points on it.
Corrupted abilities (RoP:I). While its xp-granting aspect is a road to power in any moderately-long-lasting saga, what I really like is the challenge it creates for a player: you must find a way to have your character be actively evil and spiteful in every situation, even when you, as a player, would rather have him not be. The best example in ArM5 of a V/F that shapes a character's behaviour without dictating it. In this sense, it has immense reskin potential, if you replace evil/selfish behaviour with e.g. saintliness or gluttony, possibly adjusting its value.
Pious, Major. While it may seem bland, I like it for two divergent reasons. First, like most Major Personality Flaws, it creates a lens through which the character sees the world, and thus almost a variant game: rather than the Art of Magic, you'll be playing the Art of Good Deeds. This is particularly important for magi, who at character creation can already command immense power, and for whom the big question is often what to do with it. Second, it's a source of raw mystical power for the munchkin player of a Companion in long running sagas, through Contemplative Mysticism (TC).
Restriction. This Major Hermetic Flaw has soooo many flavourful variations - and even more fairly drab ones, but who cares about those? My favourites are:
- in any scene in which you have been called by your name,
- on a target whom you love or hate - or who loves or hates you (yes, I really do love Nobilis),
- while you still have debts unpaid, unless your magic is directed at paying them.