Geargrinder is not a good champion overall, there are much better revivers. But he is the only reviver of the Ogryn faction, so you shoud keep him for faction wars.

Centurion looks interesting. I never pulled and never even noticed this champion before. His A3 looks like there should be a way to make him usefull. To be honest, I don't know wich way🤔 Ayumi and Hell Hades rank Centurion very low, but as a void he is literally rare, so I would keep him for a possible later use.

Reliquary Tender is a good champ and will help you a lot in faction wars and maybe some dungeons. I use her at Ice Golem to counter the nasty healblocking skill of the minion and for revive if something goes wrong.

Coldheart x 4

Reliquary Tender

Apothecary

Kael

Dilgol

Paragon (not leveling him)

Renegade

Bellower

Avir

Reliquary Tender helped me climb to stage 20 dungeons. I only stop using her after I got Arbiter. But then, doom tower comes out and she became usable again in Nether Spider stages. 

Geargrinder, I 6 star this guy and helped me finish stage 21 of faction wars. That is because I have no other healer / reviver on that faction. But now, If I remember it correctly, there is a new epic champion that is much better than him. Forgot the name, but if you happen to summon another healer in that faction, you may not need to build this guy. 

If you are asking what other rares that are still use up to end game, the most common answer you will get are:

Coldheart

Apothecary, umm maybe not really in endgame but still usable in stage 21 of faction wars. 

Bellower, some still uses him as their brutal farmer

Reliquary Tender

Renegade

Fellhound, like Bellower, some still uses this campion as farmer, and secondary healer in faction wars

Pain Keeper

Geargrinder is quite interesting 

Bellower

Valarie 

Warmaiden

Reliquery Tender

Spirit Host 

All Excelent for low and Mid game , a Couple even into End game

Here are all the Rares - I have leveled up over a 2 year period playing Raid.  

i

The heroes are level 50 or higher.

I used all of these heroes once upon a time which is why they are all leveled up.

A few of these heroes I still use.

A few of these heroes I no longer use.

A few of these heroes I still need to 6 star.

Incase, you are not familiar with the hero portraits.

I will give you a list of there names and whether or not I use them any more:

- Bulwark - Don't Use

- Seducer - Use - Undead Faction Stage 20 Farm Team

- Renegade - Use - Dungeon Speed Run Team

- 3x Coldheart - Use - Spider, Fire Knight, & Doom Tower

- Bogwalker - Don't Use

- Warmaiden - Use - Dragon & Arena Team

- Frozen Banshee - Use - Clan Boss Team

- Kael - Don't Use

- Apothecary - Use - Arena Team & High Elf Faction Stage 20 Farm Team

- Pain Keeper - Use - Dark Elf Faction - Not sure if she will stay on my Farm Team or Not

- Bellower - Use - Ogryn Tribe Faction - Not sure if he will stay on my Farm Team or Not

- Fellhound - Use - Demon Spawn Faction - Not sure if he will stay on my Farm Team or Not

- Veteran - Don't Use

- Doomscreech - Don't Use

- Wretch - Don't Use

- Skullsworn - Use - Lizardmen Faction - Not sure if he will stay on my Farm Team or Not 

- Temptress - Don't Use

- Witness - Don't Use

- Athel - Don't Use

- Executioner - Don't Use

- Spirithost - Use -  Dark Elf Faction - Not sure if she will stay on my Farm Team or Not 

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I own Reliquary Tender, but I pulled her from a shard very late.

By the time I got RT, I had Mausoleum Mage & Doom Priest for Cleansing.

By the time I got RT. I had Gorgorab & Arbiter for Resurrection.

My Reliquary Tender got overshadowed.

My Reliquary Tender never had a chance to spread her wings.

I use Mausoleum Mage against the Nether Spider in Doom Tower to cleanse the Poison.

I use Doom Priest against the Magma Dragon in Doom Tower to cleanse the Hex.

I was able to beat all of High Elf Faction getting the full 63 points with out RT.

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The last thing I want to say is don't underestimate Rares or be in a rush to get rid of them.

Faction Wars is very expensive.

- 13 Factions in total

- Each Faction team using 5 heroes

Most people are not using Full Legendary Faction War teams.

A Full Legendary Team Fully booked is way to expensive.

Each Legendary Tome sells for like 25$ a pop.

You don't have to be a math expert to see how fast the numbers will add up.

Hopefully, You can understand why I still use some Rare heroes in my Faction War teams.

It can be a lot cheaper to Skill Up a Rare hero vs. Legendary hero.

I, personally, haven't finished all of Faction Wars.

I am still only half way, but you can see how I am slowly adding in Rares to my team.

It is something worth thinking about.

Rare Ranking:

1. Apothecary

2. Kael

3. Warmaiden

4. Coffinsmasher

I would 60 all of these before Coldheart as a new player.  Spirithost should be in your arsenal at 40 or 50 depending on who you pull along the way.

Even if "what are the best xx champs of certain rarity" is the most common issue on youtube about Raid, I think it's hard to give such a ranking in an abstract way, without knowing the player's full roster.

Of course Kael is a good champ. He can work as arena nuker and as poisoner at the Clanboss. But if you have a Frozen Banshee or Occult Brawler for Clanboss, you don't need Kael anymore as poisoner. And, looking at her self-buff with 50% atk, Dark Elhain, wich you get as free login reward, is probably the better arena nuker for most players. With her 25% atk buff as argument even Athel is a better arena nuker than Kael for most players.

Warmaiden is a good AoE def debuff champ. But if you are lucky enough to get Tayrel or Stag Knight, you don't need her anymore.

Coffinsmasher is a good atk debuff champ for Clanboss. But if you get Peydma or Jareg, you don't need him anymore.

A lot of rares are usefull. Finally it depends on your team composition. What I wrote above about Kael as worse than Dark Elhain or Athel in arena is not a fact anymore if you use Seeker or Gorgorab in your arena team.