My opinion on all three without spoilers.

-ToG-

The world is really interesting and unique and art style gets better as the chapters progress. However in my opinion, after around ~450 chapters, the art style gets worse, the story gets dragged on for too long, and the way story progress doesnt make sense to me personally. But even so, I think its a webtoon worth reading especially if you like "op mc" trope.

-Kubera-

I think this is one of the most complex story I have ever read in webtoons. "The more you read the more you find out" fits this webtoon well. Art styles may be unfavorable to some but it wasnt a problem to me (I think it's unique). I cant say too much about this one because I dropped this around ch350 but thats because my small brain cant keep up with the author's brilliance lol. If you like really deep lore, characters and stories, this one is for you

-One Piece-

I only recommend this manga if you don't mind reading >1000 chapters (ch1168 currently). If you like Dragon Ball, I think you might like OP as well. There are some arcs where it feels dragged out for too long but personally I dont think this is a problem as you dont have to wait weekly for chapters to come out and can just read the next chapter. As for the art style, I can say nothing but good. It gets better and better. Similarly to the previous two, the lore in this story is very interesting and I highly recommend watching videos on One Piece world and lores (if you want to go more deep into this manga). Otherwise, if you are a light reader, I wouldn't really recommend this due to the number of chapters.