C6 ranking

This is focusing on the C6 itself, rather than assessing C0, C1, C2, ..., C6 altogether. For example, Raiden's C2/C3 is arguably more powerful than many C6 characters, but Raiden's C6 itself may not be as big an increase as her C3. But that does not change the fact that a C6 Raiden is just as powerful, if not more so than a C3 Raiden (although maybe not by much). Here we would be evaluating Raiden's C6 rather than a C6 Raiden.

Best C6 effects
Tier Characters
SS (get your money's worth, the best of the best) Xiao, Itto, Eula, Yae Miko, Shenhe, Faruzan
S Diluc, Dehya, Yoimiya, Raiden Shogun, Venti, Mona, Yelan, Yun Jin, Cyno, Alhaitham, Scaramouche, Nahida, Jean (one of the few dmg reduction abilities, but her C4 is more broken), Qiqi (can revive whole party), Tartaglia (C6 while good may as well be a C1 effect like C1 Kazuha), Beidou (-15% electro res), Rosaria (-20% physical res), Anemo Traveler (-20% anemo res), Kujou Sara (one of few crit dmg buffs), Gorou (one of few crit dmg buffs), Sucrose (similar to Kazuha passive), Kazuha, Xingqiu, Lyney, Focalors
A Bennett (infusion can mess up some characters), Chongyun, Xiangling (15% dmg bonus does not buff own pyronado, XL's C4 is better), Kaeya, Fischl, Thoma, Albedo (C2 is better), Yanfei, Barbara (one of the few revival abilities), Diona, Shikanoin Heizou, Yaoyao
B Amber, Noelle, Ningguang, Lisa, Geo Traveler, Electro Traveler, Sayu, Xinyan (C6 would be better for someone like Itto), Kuki Shinobu, Razor, Kokomi
F (ripoffs if you need to spend money for this) Hu Tao, Ayaka, Klee (C6 has battery properties), Keqing (C6 gives limited dmg bonus), Ayato (minimal dps increase 900/12 < 100 mv, not affected by namisen hp buff), Ganyu (C6 reduces charge time but does not increase dps by much compared to even C1, the worst C6), Zhongli (shield heals but many Zhongli simps say healing is not needed when you have a shield), Tighnari (because E already reduces charge time, C6 does not do much), Dori (but Jean's passive already does a similar thing at C0), baizhu

Other constellations

Usually C6 stands out as the most powerful constellation in the kit. But sometimes earlier constellations can be very powerful as well. Some highlights include C2/C3 Raiden (ignore 60% def), C4 Jean (-40% anemo res), Chongyun's C2 (reducing cd by 15% within e), Xinyan's C2 (100% crit rate increase on q).

Note Aloy does not have constellations.

Overall constellation rankings

Ranks the 6 constellations as a whole: note that constellations are only one part of the character. A character can still be great at c0 even if their constellations are not as great.

'Good' means the designers deliberately did not make the constellations as good as other characters and that the constellations could be further improved from a design perspective. The constellations are definitely good and better than average/standard, and pulling for them will definitely increase the strength and value of the character, but one can be in a position to ask the developers why they did not make them better. Constellation sets that only give stat buffs (crit, atk, dmg bonus) may fall in this category.

'Great' is an ideal rating. It can be achieved by several characters, and one can be very happy pulling constellations for these characters without much regret. Some of the constellations may even be broken.

'Excellent' means there is a broken constellation (or multiple) that sets a power ceiling and demonstrates leadership of the character. This rating is not idly given out.

Best constellations overall
Rating Characters
Excellent Xiao (C6 compensates for all the others), Itto
Great Eula, Nahida, Yae Miko, Yelan, Raiden (C2 compensates for others), Shenhe, Faruzan
Good Dehya, Cyno, Alhaitham, Nilou, Scaramouche, Venti, Jean (C4), Kazuha, Lyney, Xingqiu, Xiangling, Bennett, Noctis, Wriothesley, Furina (Yelan's C4 buffs team hp, Furina's C2 buffs own hp), Xiaogan, Xianyun
Standard Most other characters

Key constellations

It is not necessarily true that C2 is the best or most important constellation.

And then there are people who say Cyno and Alhaitham's most important constellation is Nahida's C2.

Characters that do not need constellations to do their job (their key / most important skill is at c0) Zhongli, Albedo, Tartaglia, baizhu, Kokomi