A lot of the time, which throw you use depends on the situation. Opponent's char, opponent's percent, stage position, and gauging how they will react to each throw all goes into choosing which one. The best way is to experiment, but here's what I'll uuuuusually do.
Floaties - You often honestly can't do much to floaties from a throw. Dthrow can be nice at low percents, as you can usually follow with a nair, another aerial, or occasionally techchase. Watch out for some floaties to try to nair you out of this though, as you won't have enough time to do anything other than sheild really before their nair comes out. If I think they're going to do this, though, I will occasionally dthrow, sheild asap, and then usmash out of shield after their aerial hits my shield. Bthrow works well to get them off the stage, and a select few you can pseudo-chainthrow at low percents with fthrow. Uthrow doesn't often lead to much, but you can pretty much always hit with a uair after it until highish percents, so you can use that to uthrow->uair spike->follow.
FFers you're going to want to uthrow most often, as it leads to aerial combos, or usmash, or regrab if there aren't platforms, or a platform techchase option if there are. Bthrow is useful as well to get them off the stage and set up edgeguards. Fthrow occasionally serves to get them off the stage as well if you happen to grab them near the edge while facing it (but uthrow would usually be better), and dthrow only really sets up unreliable techases, so should be used sparingly.
Against sheik, I like pretty much every throw, but probably use bthrow and uthrow the most. Uthrow sets up platform techchases and some simple aerial combos (I looooove uthrow->dair). Bthrow again sets up edgeguards. Fthrow is weird and i usually use it to trip up sheiks after i've done other throws for a while, as fthrow->regrab can work if they're just focused on DIing the expected uthrow. Then you just uthrow them next as they attempt to DI the fthrow, and they get all mixed up. Dthrow I don't use much, but again it can set up techchases and sheik's tech is somewhat punishable, so it can net me a usmash or fsmash if I read right.