Luigi and Diddy are both contenders, and while Luigi has guaranteed moves at higher percents, he can't kill unless he reads a DI and lands a b-air/ a lucky up-b. Diddy on the other hand has d-throw to u-air (which kills at 80% maybe less light characters and 90%-100% maybe less average), and if you DI away you eat a f-air/b-air all the time, which can also kill.
Diddy has all of his aerials, all of his smashes (but d-smash is stilla good kill move, it isn't reliable), and a kill throw. Diddy also has plenty of kill setups.
Diddy's spike is one of the better spikes in the game, and is around on par with DK's (no lie, someone made a list a while back and Diddy's was ranked 1 below DK, who was 2nd.
Yoshi's approaches insist of camping eggs and not approaching (lol), N-air in the sky, dash attack, punish and jab, and more. Diddy has banana, side-b attack, side-b grab, any sort of grab, SHFF N-air, u-air, f-air, and more.
Yoshi's f-air is so easy to punish that it is laughable, same with d-air, u-air is fine tho.
It isn't that hard to gimp a yoshi, he doesn't have infinite armor. I can d-air a yoshi and if all my hits connects the superarmor leaves. If you get a spike or a footstool on Yoshi, just call it GG. They can't mke it back. Even if they do make it back, get some free damage in.