I'm still waiting to see if any of them announces Fern and Stark to go with Frieren, since I'll probably want all 3. But out of those announced so far, Claynel's is the one that I liked the most.
As far as brand goes, in my experience all of the five above are very good but none of them are exceptional. GSC is the one with the best reputation, with the most popular and well made figures, but they still have their misses from time to time (and it's getting more frequent lately). Claynel is the newest one out of these with very few figures already released, but those few have pretty good quality, very close to the prototype. Like said above, Koto's rarely messes up, you'll get what you paid for, something close to the prototype though not perfect. MegaHouse is weird... sometimes they release amazing products, sometimes they look like overpriced prize figures, but usually they are very close to the prototype show, though still very expensive. I like Ques Q a lot, but who knows when they will show their Frieren (and if they will release it one day).
Anyway, tl;dr, they are all trust worthy, if you love the prototype you'll most likely also love the final figure. Just get what you think makes more sense for your budget, and when in doubt you can always wait until they are released so you can choose based on pictures/videos of the final products.