Yuusha

I like this one. Just get that out of the way. I liked it when I first heard it on the series. The video just cemented that opinion.

The song is up-beat, but the lyrics are kind of melancholy. It's appropriate for the series (and the manga), which is sad and funny and, occasionally, fairly action-packed. The video is beautifully put together, focusing on Frieren's original hero party more than the trio she's part of during the series. This fits with the lyrics of the song.
Altogether, a nice package. I'd probably listen to this if it was just a random song, to be honest. As an ... OP for Frieren, it works well.

this opening is not just any opening, it represents a love letter from frieren to himmel or what she feels when she thinks about him.

apart from the anime, this opening lets us see what frieren is really like.

and how he expresses what he feels when he remembers his group and most of all, Himmel.
everything that frieren does, collecting grimoires, adventuring and collecting magics

apart from adventuring and collecting magic because he is passionate about it and it is his "hobby".

he does everything for his loved ones, for his group and most of all for himmel.
...
frieren has very nice feelings even if she is very serious and cold.

everything he does is for the moments and most of all for the lesson that his group left him and what himmel was for frieren.

sincerely a very nice opening and with feelings well captured.

it has a very nice lyrics and depth

sincerely the best opening I've seen this year.

Honestly, for a Yoasobi song, this does exactly what you expected: a catchy beat, interesting chants and lyrics that perfectly captures the themes of the show.

However, when you consider the nature of Sousou Frieren, which is an anime that does not follow the usual fast-paced and excitement of its fellow fantasies and is more of a tranquil and slow journey like Violet Evergarden, the OP really feels out of place here. A rare case for Yoasobi as the band usually has perfect ops or eds for the respective anime (most recently Oshi no Ko or The Witch from Mercury). Compare that to the ED by ... milet, which suits the show way better than the OP.

All in all, while Yuusha is still a solid song and definitely something which can be jammed to, it is just not the right for the atmosphere of vibes of Sousou Frieren.