It's a pretty hard choice between Nagisa and Honoka. It's not that Hikari is a bad character, she's great, and shiny, and luminou--OH, you know what I mean. The thing is, she didn't have the same screen time as the other two, and when she appeared, it was more like a support character in battle.
Pretty Cure balanced wonderfully the personalities of Cure Black and Cure White: Nagisa is more aggresive, but passionate, like a warrior, and Honoka is the mind of the team, wise and more calmed. They perfectly match the other... but Hikari is like a third wheel that I'm not sure what could represent. She might be the hope of the team, the one that shows up when everything seems lost. I dunno.