Which constellation is worth it?

The one on the character you'll actually use. The damage increase isn't big enough to be stressing over and the event is year round so it's not like you'll miss out.

Get a C7 for Masterless Stella Fortuna.

Casual C6 Ganyu and Childe flex. Wanderer maybe? Or play the long game and build towards a C2 to want.

Try Klee and her new upgrades?

Wanderer, Klee, maybe Ayaka if you wanna gamble on them letting us re-pick in future years for C2, or just go for C7s and improve the characters you actually use.

Whichever one of them you use the most, grab their cons. Though considering Klee's current upgrades, she'd be worth your while. Otherwise, choose Scara's or Ayaka's, and maybe Tighnari (but only if you use him a lot, from my experience, I use him on one of my teams so I actually took his cons).

Any character that has C6, you get masterless stella fortuna with a free 5 pulls.

If you only actually use childe and he’s already c6, I’d personally just get the Stella fortuna

Only Klee

Keqing so you can level her up to 95 or 100.

Aggravate gets an unusually high bonus from level increase, because it is the only reaction that gets the reaction buff and the defense buff.

(overloaded gets the reaction buff but not the defense buff, melt gets the defense buff but not the reaction buff)

there's no requirement that you have to use the golden stella on the same character you earned it from, so grab any of the c6 ones so you can level up a character you actually like instead. it's like 1 to get lvl95 and 2 of 'em to reach lvl100.

If you don't use any might as well go for C7 and grab the masterless fortuna to make someone level 95 and 100 down the line.