The madVR Envy Extreme offers cutting-edge image processing, combined with HDR dynamic tone mapping, auto aspect ratio control and 3D LUT calibrations. It's not cheap, but the performance is exceptional.
The HDFury Vertex is technically an HDMI splitter and scaler, but that description simply doesn't do this AV Swiss Army Knife justice. Is there any HDMI issue it can't fix?
If you want to get the colours on your display as accurate as possible but don't want to break the bank, Murideo's Prisma might be the video processor for you.