Falcon- Retreating sh fairs, sh bairs out of dash dance. f-tilts can be nice too. Also, mindlessly dash dancing with falcon is useless. You need to build your own rythms but make sure they aren't patterned, learn to sh fair right out of a nice tight dash dance
Fox- Retreating fairs, bair is a nice option too but you need to be SH z-cancelling these like 99% of the time or else you get punished. if you want to be campy, get shdl down. Again, some nice f-tilts worked in for a surprise, and d-tilts can be nice because of the range. You can tech chase pple with fox very annoyingly working in jabs, f-tilts other sh*t and then just running Usmash them...
Mario- Retreating fairs again, fireballs are cash money. try to get it down so you are pressuring as the fireball arrives. Sh-uairs are a nice surprise because it comes out nice and quick and this can lead to multiple things such as u-tilt continuations into more uairs and so on... utilizing mario's dash attack can be a nice surprise at the RIGHT %s. Doing it too early gets you grabbed n sh*t.
PIka- fair is only good near the ground to possible lead into a grab/utilt depending on where your opponent DIs. Uair is pure cash tho. Great range. Bair is good but some people tend to abuse it and play backwards the whole time which is unneccessary. if you are playing taller chars... sh dairs are nice and you can keep tech chasing them with it. learning to sh dair into a Usmash at low percents is a nice combo starter. as you start playing better players this becomes important.
Jiggs- abusing Nair is effective. pounds can surprise people who dont understand its range. teleporting is difficult to learn but when you do lead it into fairs. try to get the hang of fair-nair to grab-->****
Ok, those are just my mains, and that was kind of all over the place. As people above have said, it depends on your opponent, their playstyle and their char. If you play someone enough to know their strengths you can start developing strats to limit them. The above are just general guidlines to get you started man
You can work in some tricks too, spacing is about mindgames as well. Working in d-tilts just after z-cancelling an aerial with pika is just an example. that split second you are on the ground is when alot of pple will come in and try to grab you... you need to bait them into lowering their defences by making them think you are vulnerable. try to get inside your opponents head.
Edit: Being able to control the aerial movement of your char during an attack is an integral part of spacing, especially with fairs. Retreating, advancing, advancing to retreating, stationary... this control is important. Overall knowledge of the game physics/hitboxes as was said above is naturally imperative.