chrom: S. rightful king and tons of abilities to utilize it with fred: S pavgis+merc line. need I say more? virion: C. virion's mods are not really that special, he's basically a throw away father since other ones provide better mods but are needed elsewhere.
Inigo doesn't benefit much from RK. Most of Inigo's set will be using RK for 1 skill (Armsthrift doesnt count) LB, Proc, AT, Galeforce/Vantage is what the standard Inigo runs. With rallies and etc the Proc's rate is high enough. RK wastes a slot for uneccessary overkill on that proc. This is very overrated especially when you can compare to some of the other options he have that make his skillsets even better. Only reason this gets A and not B is b/c his stats are ok. Astra is a poor proc compared to Luna/Sol so the only thing RK is useful for is Lethality. Its still very luck based though and you will most likely build a set around it for the few times it will activate.
Frederick!Inigo = S
That skillset is amazing. Its like the male version of Donnel!Kjelle with more Str/Spd/Skl but less Spd. Its only - mod is Magic.
Virion!Inigo = A
Those mods are great allowing Inigo to perform many different offensive roles. A great Galeforcer due to gaining Lifetaker as well as more Faires to complement. Virion isn't a waste on Inigo as nobody really wants him.
Wow we got probably 3/4 of the best/most discussed fathers in the first poll :o --- Dry Bones+Magikruiser FTW in MKW ^-^
Actually, is it too late for me to downgrade my vote for Chrom!Inigo to A? Upon further reflection the thing that basically edges it up is Rightful King,(which is situational in the postgame) and while his class sets are good, Virion and Fred both give Inigo 1 more class, so I don't think it's really fair for me to prioritize Rightful King and mods over an extra class set worth of skills, so I'd place them on equal parity.
I must admit, my first run where I used Fred!Inigo, it was at the time where I didn't really use the pairings properly, and Chrom!Inigo kind of spoiled me the next few times around.
But, he will be stuck with Astra for big damage, and that's not very good. B rank.
Imo that only matters for TSON since he has Sol as a viable proc for every other map, and Astra isn't as terrible as people make it out to be for an offensive proc in that setting. He'll probably end up in the support role more often for TSON to take greater advantage of his dual strikes, and with access to Aggressor on top of one of his faires, he still makes an amazing enemy sweeper in the lead role.