Overview
In a nutshell
Diddy Kong would just be another agile lightweight mid-tier if it weren't for his bananas. Diddy's metagame revolves around his self-generated bananas. While he isn't the only character who can generate items by himself, his bananas have the unique property of forcing a tripping motion on the ground. This property allows him to follow a banana 
on a grounded opponent with anything he pleases, such as a grab or a smash attack, while hitting airborne opponents can force an air dodge or chip on some damage. His bananas also give him amazing stage control, greatly limiting his opponent's options. Bananas are not his only tool either, as his Peanut Popgun projectile, while weak, travels in an erratic fashion making it difficult to dodge, and his Monkey Flip is a good mixup and approach move. He also has good agility and his moveset has comparatively great comboability. In all, learning to utilize his bananas combined with his other tools while avoiding opponents using them against him is the fundamental to playing a deadly item-based character that is Diddy Kong. He currently ranks 2nd (A tier) on the tier list (the highest-ranked of any Mario character including sub-series characters). This is, of course, thanks to his solid Bananas which can penetrate even the tightest defenses and being one of Meta Knight's few difficult matchups (having an even matchup with him). As a result, he has many quick attacks, good agility, a solid down aerial meteor smash, good grab range, powerful throws for his size, and an impressive number of tournament wins.