Down-B spike is slick. I love it.
I also like offstage n-air and b-air. Stage spikes can work at the right damage, and both come out pretty fast.
I think of F-air as a stomp that happens in front of you instead of below you. So as they recover, you can choose between F/D-air based on your vertical relation to them. F-air is also a little slower, so be careful of that. Both have punishable landing lag, but if they're recovering to you, you've got enough control over timing/pace.
Sort of off topic, sorta not: As ledge game goes, you've gotta predict as always. If they're gonna roll, you can grab or down-B for a punish. If they like to jump, you can time an aerial (or even a u-tilt) to interrupt the jump and maybe spike. If they attack on the way up, your best bet is probably the simple shield->punish. If anybody's got other strategies, do tell.