I think there was a topic about this somewhere but oh well....
I found the best counter-pick stage to be Dreamland64. No, you won't get the ~50% KOs you might get on YS, but I'd argue you get much more protection from Fox's vertical KOs. When Fox goes in for a u-throw, just DI left or right, since he's probably going for the u-air. Smash the stick sideways when he goes for a u-smash too (you'll do it accidently at first, but with enough practice you'll have a fair amount of influence on your trajectory). When fox isn't u-throw/u-air/u-smash, you can DI up for about 90% of his other attacks (d-tilt and u-tilt being the only real exceptions). Smash the stick fast enough, and you'll only die to u-throw -> u-air (which still kills below 100% sadly), and shine-spikes while recovering (although, even these can be recovered from if he doesn't hit you while you're under the stage).
Now for attacking fox (the real fun part). If his tech skill is somewhat lacking, you can probably get away with shield grabbing his aerials, and then d-throw to stomp (my personal favorite to pull, since if they don't know how to fight 'dorf you can stomp them several more times, or end it in a f-air). If he's shffling perfectlyl and following with a shine, however, that option is closed to you. Please don't turtle up if you face a technically sound fox. Instead, keep moving and search for an opening (whiffed attacks for example). Then go for a JC grab and follow up with a combo from there.
Speaking of combos, ganon has a few decent combos. The most spectacular (and, inevitably, the hardest to pull off) start at around 0%. As mentioned earlier, d-throw -> is great when it lands, but asks for punishment when it doesn't. Instead, try u-throws at lower percents (followed by an aerial of tilt of course), and switch to d-throws at mid-high %s. While throws are often the main way in which ganon gets his combos in, he has others. F-b is one alternative to throws; it sends fox upwards while dealing a refreshing 15% damage, but it has some nasty lag if your foe dodges. D-smash is similar to f-b (good laucher, high damage, bad lag), but it can lead into by another combo starter (u-throw at low percents I believe). The STOMP is a another risky alternative, sharing many of the traits his f-b/d-smash have. Finally, d-tilt is a rather slow attack, but used sparingly in conjunction with the wavedash I find that I can often pop up slippery foes at just the right height for a f-air.
This isn't really a combo, but another thing I enjoy doing at low percecnts is just jabbing fox 2 or three times at the beginning of each of his stock if he gets too aggressive and predictable. Nothing like a free 20% to start the match.
Also edge-gaurd fox. After you knock him off, get ready to intercept/punish his side-b if he's above the stage, or grab the ledge and u-air him if he's below. If you're waiting while he's charging up his firefox, stall at the ledge (grab ledge -> drop -> re-grab in quick succession), to get some invincibility while you edge-guard. Don't let him back on the stage. Think of letting him back on as him getting a new stock, but you lose out on some of your two-hit combos.
Hope this helps.