Well here's an easy to execute true combo.
Monado Jump: U-throw -> Air Slash. It works against characters who fall faster. Theoretical basis for this combo is that the fall speed does not effect the length of distance a character is sent while in that animation where no action can be performed (preventing air dodges etc). Knockback effects this, but fall speed is an outside factor.
DI plays a huge factor in this combo. There are many cases where if the opponent does not DI, he will not be effected by this combo and therefore it is not a true combo. Furthermore, opponents' DI usually make the combo work for much greater percents. For consistency, I am only listing the percents for no DI. For this reason, in actual gameplay this combo has wider ranges of use.
---If the opponent DI's towards you, he will end up behind you, in which case you must reverse air slash (there is enough time to realize and react to their DI in this situation).
---(same issue as DDD) is a situation where the game is saying that the combo broke between the first and second strike of Air Slash, even though the second hit still landed. I'm unsure of if this is a mistake or if there actually is a way for the opponent to get out of the combo at that point.
Fox: 82-93%
King DDD: Only works if he DI's (roughly 85-128% if he does DI)
Falco: 83-87%
Greninja: Only works if he DI's
Monado Jump Shulk: 84--99%
Captain Falcon: 96-110% (really messy testing... can work as early as 71% if opponent DIs forward apparently).
Diddy Kong: 89-97%
Sheik: 85%
Little Mac: 77-93%
Mega Man: 85-102%*
Zero Suit Samus: 71-78* 79-110% (later percents can kill!)
Ganondorf: 90-111%*
Meta Knight: Only works if he DI's
Donkey Kong: 103%*
Ike: 88-101%*
Rob: Only works if he DI's
Wario: Only works if he DI's
Bowser Jr.: 95-105%*
Duck Hunt: Only works if he DI's
Link: Only works if he DI's
Lucario: 92-102%*
Lucina: 87-93%
Marth: 87-93%
Pikachu: Only works if he DI's
Dark Pit: Only works if he DI's
Bowser: Only works if he DI's
Seemingly everyone else (Mario, Shulk, Jiggs, Charizard are what I tested): Only works if they DI or does not work at all
If there is a * next to a percent (for instance with boswer jr.) it means the combo counter does not recognize the first and second strike of air slash as comboing even though they do hit. For Mega Man, at the lower percents he can input down right after the first strike of air slash to avoid the second, but at higher percents he is still hit. ZSS cannot drop out using this method, but it still claims the two strikes do not connect together.
I list/test the characters in the order of their fall speeds according to this:
http://smashboards.com/threads/smash-4-aerial-speed-list-horizontal-fall-and-fastfall.382495/