Okk.
First off lets list all of Pikachus Kill moves:
1. N-air
2. Thunder.
3. Last Hitbox of d-smash
4. F-smash
5.Up-smash.
6. Off stage d-air
now First off N-air is one of pikachus best moves. I personally use it to rack-up damage and to gimp. And i also use it to follow-up like u-airs, and u-tilts. N-air is a great move. Great for killing(if fresh) and great for racking up damage.
2. Thunder: I would say that this move is Piakchu signature move. Everyone knows pikachu for this move.
Thunder can be very hard to get a hit with, but then again, it has great knock-back, and great vertical range and great damage. Ether set-up with u-tilt. or up-smash. or do what i sometimes do. When your opponent hits you up into the air, and your up in the air were they cant see you but that little bubble shows were you are. use thunder, but move pikachu out of the way. so the thunder goes straight down to hit your opponent.
3. The last hit-box of d-smash, Time your d-smash just right so only the last 2 hitboxes hit. around 140-160% your opponent should die. That is if d-smash is fresh
4. F-smash. I use this smash as a damage rackerr. But it can also be used at a great kill move.
5. up-smash. Up-smash can be used as a kill move. But all pikas know the fthrow<dash<up-smash combo, so i dought up-smash would be fresh, but up-smash is one for pikachus fastest moves for the hitboxes to come out. and It is a great kill move if fresh
so bottom line. It really depends on your playing style for pika's "best kill move" If you use n-air for damage racking then it wont be your "best kill move" If you use it for killing then yeah, it would be.
Same does for every other kill move i listed up there. It really depends on the play style and the moves you use while racking up damage.