I am in the US. I use a Tidy Cats clay litter now, and have tried almost all the others. I like World’s Best except for the price, and I don’t find Dr. Elseys to get any heavier than other clays. I don’t like the lightweight litter because it seems very dusty. I prefer a clumping to the others - unless I have young kittens, and then I need to use a natural, non-clumping, litter in case they eat it.
I used to buy litters that seemed more environmentally-friendly (paper, corn, etc) until I read this article: https://www.nytimes.com/wirecutter/reviews/best-cat-litter/#sustainability-and-cat-litter.
Cat feces should never be flushed down the toilet, though it is very convenient. It can introduce parasites (toxoplasma) into the water system and not all filtation systems can eliminate it. https://www.wusa9.com/article/news/verify-yes-flushing-cat-litter-is-helping-to-spread-a-parasite-in-drinking-water/507-038b8ccc-5cb2-49ba-a8db-bbe687c84618