Best GPU for athlon 5150 apu

The Socket AM1, previously named FS1b, is a socket designed by AMD, launched in April 2014 for desktop system on a chip in the value segment. Socket AM1 is intended for a class of CPUs that contain both an integrated GPU and a chipset, essentially forming a complete system on a chip implementation.

I have a PC based on AM1 socket and it has a single PCIe x16 slot that runs at x4 making it unsuitable for a discrete video card.

Your Athlon 5150 APU has:
Integrated graphics GPU Type: Radeon R3 series
Microarchitecture: Sea Islands
Shader cores: 128
Base frequency (MHz): 600
The number of supported displays: 2

What your motherboard is or if it supports a video card is not known.