I'll take that, gimme a sec…
When you have an entire orchestra set up ready to combo out, and need an extra power to do so, badly enough to bleed for it, Polyphoric flare is the best card in Argent's deck.
When your allies want their best stuff back the turn after their next turn, Vernal Sonata is the best card in Argent's deck. (The 1 HP is generally just gravy, save perhaps if Scholar is in the game.)
When 1 HP every turn isn't just gravy (or Scholar is in the game), Rhapsody of Vigor is the best card in Argent's deck.
When 2 HP every turn would be even more not-just-gravy, or when you want to do something resembling actual damage, possibly while getting a Blinding Speed effect (or two, if you give up the damage part) every turn, Drake's Pipes is the best card in Argent's deck.
When your teammates are a lot more helpful than you are, you can get multiple Accompanies a turn, you have original Nightmist , Omnitron-X, or The Scholar on your team, or you're playing a slower-rolling villain which you'll need long-term help defeating, Inventive Preparation is the best card in Argent's deck.
When you have a bonus to damage or are teamed up with Absolute Zero (I have to remember to try this!), Scherzo of Frost and Flame is the best card in Argent's deck.
When you don't have another instrument, Eydisiar's Horn is the best card in Argent's deck (assuming it's not your first turn and you didn't just get wiped).
When you don't have an Instrument, want to have another Instrument, or would like to increase your hand size, Instrumental Conjuration is the best card in Argent's deck.
When you feel like being weird and doing your Accompany first, Anpuku's Drum is the best card in Argent's deck.
When a lot of 1-damage instances are flying around, the highest or lowest HP hero is taking a lot of hits, Nightmist or some similar hero needs every card they can get, or you can get multiple Rhythm accompanies in a turn, Counterpoint Bulwark is the best card in Argent's deck.
When you have two different kinds of songs that are worth playing, neither of which is a Melody, Telamon's Lyra is the best card in Argent's deck.
When any of your teammates is doing a lot of little damage pings, and/or can boost little-damage pings into something significant, Syncopated Onslaught is the best card in Argent's deck.
When you need a versatile and reusable tool to solve a wide variety of situations that don't involve dealing damage, Sarabande of Destruction is the best card in Argent's deck.
When you want to be able to play any damn Accompany you want, Xu's Bell is the best card in Argent's deck.
When what you need is the best Instrument in Argent's deck, Musaragni's Harp is the best card in Argent's deck. (This is particularly true on turn 2, when it and Xu's Bell are the only cards that will let you get both halves of whatever song you played on turn 1, unless you opened on Subdominant and another song, which might let you use the Drum or Horn effectively. Of the two, the Bell is slightly less likely to be optimal, despite being apparently stronger, since Rhythm performs are weaker than Harmonies on average, though it will always depend on the situation.)
When THAT HAS TO DIE RIGHT NOW, or when you can get one or more Harmony-Accompanies and don't badly need to heal, Cedistic Dissonant is the best card in Argent's deck.
Whenever you have a non-Melody song in play, Silver Shadow is the best card in Argent's deck. Even if you don't, it's pretty damn good.
When you need to play more cards in the early game, or combo out in the late game, Alacritous Subdominant is the best card in Argent's deck.
The rest of the time, Inspiring Supertonic is the best card in Argent's deck. I mean, it gives you more powers to play all the other songs with, so it kind of can't help but be the very best in most situations.
At all times, Arcane Cadence is tied with the other best card in Argent's deck, since it gives you roughly a 1/2 to 1/4 chance of being that card, plus letting you draw another card and stack your deck a little.