Probably Marth. His range is longer than Falco's, who is Diddy Kong's weaker match-up. A longer range is pretty much a victory over a short range character.
Now for my discussion on Diddy Kong:
Diddy Kong is a very good character. I used to use him a lot, before Meta Knight.
Contrary to popular belief, his banana peel is very useful. If someone is running toward you, you can use it and they will trip. Now here is the trick. You should be able to make them trip multiple times with just one banana peel. They will keep getting up and they will trip again. I do this against other people all the time. If someone does this to you, then recover from the trip opposite the banana peel. Now that you guys know this, you probably won't find this useful, so plant a banana peel behind them too, but make sure that it is in a position where they will trip from recovery.
His up smash isn't very powerful, but his side smash and down smash can send opponents flying at even 70%. His popgun is kind of stupid, but if you hold the shield button in the beginning of it, it will cancel. This is of course called popgun cancel. It works against people that shield often, but still isn't very effective. He has three different spikes: his up b, his side b, and his Dair. his up b only works as a spike from the bottom of the jet blast. If you use his side b, then once it latches on to someone, then press up on the control stick and it will send them straight down. These two spikes are hard to pull off though, so just use his Dair as a spike. His side attacks are pretty strong too.
Overall, Diddy Kong is a good character and I will expect him to probably get high tier.