Hero's Crest a Keyblade specific to Kingdom Hearts II, and is unlocked after defeating the Hydra in Olympus Coliseum. The design of it resembles the architecture of the Coliseum itself, with the Keychain referring to the coin that allows for Sora to explore the Underworld unaffected.
Besides a great look, it also has a range of other positives. The strength of Hero's Crest - considering how early in the game you can unlock it - is very high when compared to other Keyblades throughout the story.
The ability attached to it is also very helpful for early game worlds: Air Combo Boost allows you to increase the strength of the finishing attack in Sora's mid-air attack, which is especially helpful here when you don't have a lot of extenders otherwise.
Despite being great in many regards, Hero's Crest does have its downsides - namely that it has very low magic power, and quickly becomes outclassed by other Keyblades in the game. Although these are problematic later on, Hero's Crest is one of the best options when you immediately unlock it.