I list all my 6-star champions in order of 6-starring and rate my satisfaction with them, where 5 is fully satisfied and 1 is the opposite. 

The first score is how it was immediately after 6-starring them (well, how I think I should have felt), the second score is how useful a champion is now.

This score is not how good champions are, it's how I felt after 6-starring them: it's subjective and account-dependent.

1. Elhain 5/1

Campaign farmer and everything else in early game. She was my DPS in High Elf FW-21, and stays in Tag Arena until I train more champions.

2. Vrask 5/3

A perfect healer for early-mid game, who carried me to Arbiter. One trick pony, so once he is not mandatory, he is not that useful. He helped me in Doom Tower Hard floors 44-45, so he has some uses still.

3. Tayrel 5/5

Everything you want in one champion. A staple of every team. Literally every team in the game except solo campaign farm.

4. Doompriest 4/4

Situational. Great in Ice Golem and where you need single debuffs removed. Not that great against many debuffs like Bomb floors of Doom Tower. I don't have a clan boss team where she is needed. Heal is a nice bonus, but it's not big.

5. Coldheart 3/4

Keeping her alive is difficult, so she was not a game changer when I leveled her. Much better with Arbiter. I don't give her 5, because I'm not fully satisfied yet. FK runs sometimes fail because she throws A3 and A2 into a shield. I still can't beat Spider-20 on auto with acceptable win rate, so while she is great, she is not "I beat Spider alone". I'm happy that I have her and sure she'll be 5, but not yet.

6. Luria 3/3

Low scores because she refuses to strip unkillables with A3 on auto. She is in my arena team, but is not mandatory there. I used her to beat Doom Tower Hard floor 48 that I couldn't beat, for example, with Scyl in her place, so she has her uses... if you can restart a fight until 5 freezes land. I don't regret 6-starring and booking her, but she is not an obvious choice.

7. Skullcrown 5/5

7-9 seconds campaign farmer changes everything. If you think that 25 seconds is the same and investing in a faster farmer is not worth it, you are wrong. With Elhain it was "so many champs, so little food, another week of tedious farm". With Skullcrown I have more food than champions I want to 6-star. I've got lego book at 18k and 10k pts in champion training event and tournament. I leveled 7 6-stars in 4 months before Skullcrown and every one of them was a pain. I leveled 11 6-stars in 2 months with her. Skullcrown is also my main DPS everywhere.

8. Frozen Banshee 5/5

A must have clan boss killer. I use her in Dragon too.

9. Armiger 5/5

He is brilliant. I use him in Spider and FK. I use him against Doom Tower bosses, and he is better than Alure or Coldheart there. I am leveling a second one to replace Alure against Scarab King.

10. Skullcrusher 5/5

He immediately let me 3-key NM. Was in my first Dragon-20 and FK-20 teams. Is being used to 2-key NM. He is in my default Doom Tower team, Skullcrown kills everyone with counterattacks.

11. Arbiter 5/5

A game changer. Before her teams contain healers and tanks that take 5-8 minutes per run. With her teams contain DPS that speed runs to 2-3 minutes. Changes you from arena bot farmer to arena bot farmer farmer.

12. Bulwark 3/4

He did not give me a stable 2-key NM. He is too random. I had a team that dealt 17-19M. It changed into a team that dealt 12-24M. It is good if I abuse game closing. He is in my CB team, but he was not the one who improved it. He is my only source of HP Burn for Frost Spider, hence improvement to 4.

13. Grizzled Jarl 5/4

He pushed me over the edge of 2-key NM. It's also 5-key UNM. He improved NM damage more than I expected, from 18M to 27M. A surprising fact was that using his "AoE 1 turn block debuffs, 2 turns increase defense" move before boss first AoE is better than using it to block stun. I rate him 4, because I haven't found much use for him outside of clan boss, and he is not as mandatory as FB there.

14. Spider 2/2

Sorry, Trips, I don't like Spider. I regret spending books on him. He didn't improve my FW progress, I still can't beat stage 17. I can't beat Dark Elf secret room. His A3 cooldown is too long to use him as the only source of Def Down, Tayrel does it much better. I like his aura, but that's it. He requires every stat, so I feel that it's impossible to gear him well for me at the moment. He is like Coldheart in early game in that sense. I dumped his DPS stats, and his support is not as great as I hoped. May be I did it wrong. May be he'll shine later with better gear. But will I need him later?

15. Elegaius 3/3

Situational. I built her for Doom Tower, without books. I used her to beat floor 48. Next time I used her was floor 55, and I passed it on the first try, so I don't know if she is required there. I had better options on other floors. I don't have enough blue and white potions for Scyl because of Elegaius.

16. Alure 4/4

I don't give her 5, because she did the same job at 50. She made it so Doom Tower Hard floor 30 Scarab King never took a turn. I can't forgive her for using A2 against floor 50 Scarab King at 20 turn meter. She couldn't resist Temptation, then she got a turn of weak hits, and a team was wiped on 18th minute. I was ready for weak hits, but not for Temptation. I've built a second Armiger. Alure stayed in a team too, and she helped with both waves and the boss, but I can't credit a win to her. If I'm not sure it's 5, it's not 5. May be when I get to Nether Spider.

17. Scyl of the Drakes 4/5

She is great. But Arbiter is better. I use Scyl in Doom Tower to stun waves and revive Arbiter. There was no floor where replacing Arbiter with Scyl improved my team. When I use them together, it works, but sometimes Scyl dies on the same turn as Arbiter. I haven't found a place for her in my dungeon teams yet. May be it's the same probles as with Spider: he is overshadowed Tayrel, Scyl by Arbiter. This is what I thought during first battles. But then I started using her together with Arbiter on all floors. AoE stun is amazing. Ally protecion when she revives Arbiter or DPS is great. She deserves being fully booked to get stun chance up.

18. Armiger#2 5/5

I put my second Armiger in Destroy set getting it off Coldheart. Hard-50 Scarab King is down. I leveled Armiger to beat Scarab King, and Armiger destroyed Scarab King. Well done, Armiger.

PS. 3 out of 3 champions I 6-starred after asking "who to 6-star" on this forum are 5/5.