For most people, the slam dunk brand is Panasonic, who offer extremely well-made units across a range of prices, all of which deliver great video quality.
We wouldn't suggest buying a 4K Blu-ray player any cheaper than the lowest-priced options from Panasonic, such as the Panasonic DP-UB150/154.
At the high-end, Magnetar and Reavon are two big names – and both are essentially from the same company, so you can choose options from either brand based mostly on whatever elite connection options you want.