You have a lot of characters to choose from, but the most important thing is obviously, which character you like and enjoy playing?

Obviously you could look at one of many tierlists and go with the in theory most effective characters, but if you don't like them or their playstyle it's a shame imo.

Therefore I'd say fuck the tierlists and pick characters that provide the most fun experience to you personally.

I can still give you a few recommendations if you want:

For most teams a healer is useful, unless you're a god at dodging, so either Noelle for a more flexible healer or barbara as a dedicated healer who's also fantastic at breaking pyro shields.

Childe being a 5 star is obviously a strong character, but his playstyle requires cooldown management of his melee stance to be most effective making him a uniquely high skill ceiling character, so if you enjoy that he's great. Xiangling and Fischl are both excellent for both support and dps roles since guoba/pyronado and oz stay on the field even if you switch them out.

Beidou has the "hit me, I'll counter" playstyle which I personally love since it gives me so much satisfaction to see her " no u" in action. Ninguang seems very potent if you look at the many builds and clips on this subreddit so if a geo catalyst user is your thing or you like her design wise go for it.

TLDR:

Pick characters you actually like, don't care about tierlists too much, have fun.