A lot of plumbers swear by the Fluidmaster 400A fill valve. Terry has said that he likes the Korky 528 because, among other things, it's easy to install, very flexible in terms of the number of toilets it can work in, and can be repaired by a homeowner themselves with Terry giving instructions over the phone, and the moving parts are all contained in a $3 cap that is a one-minute job to replace. I like it in addition to those reasons because on the 528MP, which has an adjustable refill ratio, the little knob that you turn to adjust the refill is very precise and stays in place; Fluidmaster uses a slider on the rubber hose that isn't really infinitely-variable, and doesn't stay in place in my experience. In our old family home on Long Island, I have replaced the fill valves on all seven toilets with various flavors of Korky 528, and they work great. They are also a bit quieter when running than the 400A, which I know because a couple of the toilets had 400As in them when I replaced them.
On the flush valve, I think the Korky flapper is almost the universal winner. Almost every plumber carries the Korky flapper on his truck, and uses it to replace the flapper on any generic flush valve. I have seen a number of pros on here opine that the Korky flapper is better than the Fluidmaster flapper, which is more rigid and a different design. That said, if a flapper closes at the right time, reliably stops the flow and doesn't leak, it's a winner.
Now that many manufacturers have gone to 3" flush valves, a plumber can replace almost any 3" flapper with the Korky universal adjustable flapper, which means fewer parts needing to be on the truck to make instant repairs.
I like the Korky replacement flush valves, which again have gone into many toilets in our own home, because you don't need to cut them, as they have an oversized overflow riser that is twist-to-lock adjustable. They are also made of a nice, heavy plastic, which is sturdy and unlikely to crack.
But others may have different opinions, which is what makes the world go around.
One other thing that's important to me: Korky products are made in Wisconsin, and their customer service people (answering at their call center in Wisconsin) will bend over backwards to give advice or immediately mail out a replacement if a flapper fails within 5 years. They really understand customer service. And I feel I should reward that. Fluidmaster is a famous brand, but a lot of its stuff is hecho en Mexico. Not to be jingoistic, but besides the inherent quality of their products, I feel good about giving Korky my money.