Oh, how I miss the days of melee, when the knee was just-plain deadly, and you really could land a falcon punch combo, if you're a mother ****in' pro.
A pro Diddy Kong is a pain in the ass, though. You know his bananas he spawns from his Down B, right? That's the only reason why he's top tier. He can do ridiculous things you'd never imagine until you saw it done. First of all, getting hit with a banana peel (randomly spawned item or his, they're exactly the same), deals a tiny bit of damage is a guaranteed trip. When someone is tripped, there's a short amount of time they can't do ANYTHING. This leads Diddy to utilize the easiest truly unavoidable combos in the game. Throwing a banana to a grab and then throw, a smash, a tilt, a "banana lock", a "jab lock", the possibilities are huge, and there's ALWAYS one for any situation. Items are banned in tournaments for a reason; since Diddy can spawn two of a dangerous item at one time, this makes him something fierce. Not broken, but fierce.
He can also utilize multiple different "advance techniques", or techs, using the bananas. As I mentioned earlier, here's an example of a banana lock.
Yeah, Metaknight would be proud. That video above is using the "glide toss", which pushes Diddy in the direction of your choice by ****ing with game mechanics (the wavedash of Brawl, essentially). If you give a shit about it, study up here (glide tossing can be used with ANY item and almost any character, btw). If not, this video's nothing new.
There's also a "super" version of the glide toss, shown here @ 0:35, G&W glide tossing and then using oil panic to just be a dick. Can't find much details on it for some reason, though... Diddy can do this whenever he has a banana, sort of like Snake's epic DACUS going across more than half of Final Destination.
And there's a lot more Diddy can do. He's top tier because those bananas can cause some serious havoc. Without them he's barely mid-tier, but with them he has a reliable projectile, approach, combo tech, etc. etc. He can even Z-drop gimp with them for god's sake, lol. A good Diddy player is about as much hell as it is to play an epic Meta, a dedicated camper Snake, or a Falco who does nothing but spam lasers until you **** up an approach, and the Falco player can then punish the mistake.