My go-to is using the bumpers as it’s the closest to simulating a paddle shift. It also keeps the shifting inputs near the throttle and brake inputs (assuming you use triggers), and leaves most of the face buttons for whatever you find convenient.
I used to use the right analogue stick (up for upshift down for down shift) back in GT6, a little counterintuitive, but it was immersive for me at the time. GT Sport forced me out of that scheme as it had locked presets that only permitted an inverted version of that right stick setup.
Another decent one is Circle/Up, X/Down, I used that on Forza 4 since I didn’t like the feel of the 360 bumpers for shifting. It’s quick for thumb placement, but it means I have to place my e-brake and boost/DRS buttons elsewhere (normally I have e-brake on circle and DRS on X).