The "best" solution is via an Ethernet cable.
Wi fi and mains plugs both give a reduced signal and hence reduced performance.
No point in having fibre optic broadband, then reduce the signal INSIDE your house by using wi fi or mains plugs.
p.s. I also have an upstairs office right above my Virgin router and dropped a cable down through the floor/ceiling. I then "boxed" it in with plastic trunking downstairs so you hardly notice it.
I also have a 4 way "hub" (or switch) in my office so I can plug 4 devices (PCs, printers etc) into the one Ethernet cable.