Well, honestly, the best option generally is to mix it up when edgeguarding from what I know. You don't want to get too complacent with a single option, though I will say that B-air is a fairly good kill-move at higher %s, yet (From personal experience.) I find that Doc-nado (Down-B) is the most reliable because of how it lingers.
Other than that, all I can add is that dropping off the ledge and (Up-B) SJPing back is an incredibly good option for low recoverers, but rarely anything else - as it has a chance to give you a pretty nasty stage spike in the process, as well as potentially giving you the ledge invincibility if things don't neccisarily go to plan.
All in all I think it really just varies on position, so I'd keep at them all. If you can avoid using pills though, I would in most situations, as they're generally much weaker than your other options, both in damage and positioning. Take into account that as a ranged move though, this will be your only completely safe edgeguarding option, so it might be worth the use if you need to rack up some damage safely for a pullback, or space the opponent a little to give you breathing space.
Hope that helps - I don't play Doc all too often, but he's definately in my top 5, so I gave my best advice. ^^;