Keep the rares until you absolutely know that you dont, or Wont need them ...

Rough list , and Yes it is ROUGH as some of the C rank and F stuff 

works really well Early and Mid game when you dont have anything From the S or A lists

I really wouldnt Put and Books into Fs or Cs though 

and you will figure out whats food as you go along

https://ayumilove.net/raid-shadow-legends-list-of-champions-by-ranking/ 

I would Compare that to Hellhades lists though,, BUt , they are fairly close ,,

although a lot of people will disagree with some of the ratings

Most are close

• keep the rares that are used to fuse Rhazin
• the past fusion-events where never repeated so far. Even if we don't know for sure, it's very unlikely that Plarium will let us fuse Vergumkaar (just as an example) again. You don't have to keep a useless rare like Wyvernbane just because she was part of this fusion. The event is over, no way to fuse Vergumkaar anymore
• if there will come new fusion events in future, Plarium will probably, as they did in every past fusion event, bring up new (useless 🙁 ) rares that are needed to fuse the new legendaries. It will (most likeley) not happen that they take allready existing rares and make them part of a future fusion event. It's the same in the actual event. Lodric is a new epic champion needed to fuse the new legendary. The rare champions needed to fuse Lodric are new as well. No champs that existed before the new update are part of this event. So no need to keep useless rares
• but make sure the champs are really useless by taking advice of the lists FluffyPudding mentioned or asking in the forum. Especially void rares are literally "rare", as they can only be found in void shards and maybe you will never get it again. So be carefull before you use them as food

Only keep void rare.