It really depends on the situation and character. Right after an uthrow, uair is usually pretty good as it covers no tech and tech in place and leaves you with enough time to cover rolls on reaction but this doesnt work at low %'s where they hit the platform too soon so you have to be able to recognize when this is a good option. At higher %'s you can just wait for their tech option then bair them. Sometimes shine can be good if they are on one of the side platforms and roll out as it can knock them offstage and set up for an early gimp. In general just play a lot of melee and you'll go through this situation so much that you'll know what works when and be able to do the right thing most of the time.