Unless I'm mistaken, Utilt has a slightly better KO potential when you smack your opponent with it while they're in mid-air, as it takes the vertical knock back and adds it horizontally. I think I've KOed with Utilt that way a lot more then when my oponent is on the ground.
Bair's got some pretty good KO power, though it's not always easy to land. I believe you even have to sweet-spot it, making it a little tougher. I usually try to knock them up in the air somehow and then land it, rather than RARing the SHBair. If I'm lucky enough to get a grab in, Dthrow --> Bair.
I'd have to agree that Dtilt's probably the best. It's got decent range, and you can play a little Bomb mindgames to go with it. A lot of times, I lay a Bomb and DI backwards, and as the opponent rushes in, Dtilts. They should be right where the bombs at, giving you a little extra defense. From my experience, it helped preventing my opponent from shieldgrabbing me, or causes them to hold their shield up a little longer.
Even though it's probably not the best idea, personally I also rely heavily on the Charged Beam for KO. I either need to fire it at point blank, as a counter for a laggy attack, or play a few missile mindgames. The benefit of the missile mindgames is that you should be able to rack up damage while toying with them at the same time. (An example of this is to MC --> Dash in --> halt dash with Charged Beam) Just delay it a little bit so they don't know if / when you're going to fire. But the majority of me landing it is during my recovery off-stage.
