Bair is one of the best in terms of %, but the hardest to land(need to sweetspot).
Dtilt is the easiest to land and KOs quite well in comparison to the rest. Dtilt can be your reliable kill move, especially against lightweights and small targets.
Uptilt has good knockback, and can KO lower than Dtilt, but ONLY if you hit them at the correct angle. If your target is in the air and gets caught at the correct angle from the first part of the heel drop on uptilt, it usually knocks them horizontally in a powerful manner. Of course, uptilt isn't the fastest of moves- however there are times where after landing a low knockback aerial hit like fair or uair and you land immedietely into an uptilt, you can catch many characters in the uptilt. Of course, I've never seen it to be a guaranteed setup, so it really just comes down to how you improvise.
Fsmash can KO around the same percents as the rest if it hasn't been diminished- but since it isn't the best of smashes to land, you'll have to look for the right things to punish for an fsmash.
All of the above work, however my favorite two ways to KO are...
1- Chargeshot. It's not as strong as melee chargeshot, but it seems to charge faster and fire faster than before, and it still can kill. The chargeshot is good simply because it's a huge projectile KO-ing shot, so learn to charge it up just for this use to kill people with. A VERY reliable setup for a chargeshot KO is a simple Zair to chargeshot.
2- Dair. The first thing to learn is that c-stick dair is always going to fast fall you, so watch out and don't suicide yourself. The second thing to learn is that Samus's Dair actually has quite good priority(especially for a dair) and really isn't all that hard to hit with. While there is no 100% reliable setup that I know of to dair spike people, since you only need the opponent to get hit by almost only ONE dair for them to lose a stock, it's still worth it. I personally like RAR off the stage into an attempted spike, because it also has the effect of making them think I'm going for a bair and leaving me to face the stage for a possible tether recovery/ledgehog.
All in all though, besides spiking, you'll likely just not be able to kill as low of % as many other characters.