All of Yoshi's outright-kill moves are as follows, in order of usefulness IMO
U-air (Quick and relatively powerful)
U-smash (Same as U-air but harder to kill with due to tendency to overuse for dmg)
Grounded Yoshi Bomb - Hit on the way up and it can guarantee a hit. Kills at 120% or so. Easy to avoid or counter, though)
N-air (Edgeguard kill move, comes out fast, not terribly strong, though. Hard to kill with it from the stage since most Yoshi players use it a lot for dmg)
F-air (Non-sweetspotted, it has good knockback. Works best when off the stage, relatively hard to kill with on-stage due to possibility of spiking. ground-spiking isn't that great.)
F-smash (strong but slow and easy to avoid/block. Range is nothing to write home about, either.)
Aerial Yoshi Bomb (Relatively weak and easy to avoid)
D-smash (weak but can still kill at HIGH percentages. Quick enough to get a surprise kill but not effective until at least 135% or so)