I haven't been interested in any roguelike games before and I've only played Resogun from Housemarque, but Returnal caught me by surprise.
It was such an atmospheric experience, and somehow very inmersive despite it's very arcadey game mechanics and movement. And oh boy the music and sound in this game. The presentation is so well put together all around.
The gameplay turned addicting too. Starting weak and becoming stronger in every cycle and with different weapons is such a cool idea. I don't know if other roguelikes do that too, but either way it is a strike of genius gameplay loop.
I'm not sure I would say it is "the best" 1st party on PS5, but just because I've also enjoyed other games such as much, but for different reasons and in different ways. Regardless, Returnal is a 10/10 in my book and it has its own pedestal (I platinumed it and all).
I should mention that it is in the PS Plus extra games collection. As I said, I didn't have much of an interest in roguelikes (to buy one), so it is thanks to PS Plus that I got to try and enjoy Returnal. Whatever comes next from Housemarque, I'll consider buying it at launch from now on.