Hard to narrow it down to one. I'll list my 5 favorites instead.
Faves: Hardy - positive outlook, cool look, good attitude and one of the most down to earth and "believable" personalities among the cast. He isn't overly cartoonish or tropey but isn't dull or flawless either.
Sapphira - unlike Hardy sapphira is more of an escapist character but she's easily the most badass member of the cast. I love no nonsense women who can kick butt and sapphira is definitely that without crossing the line into tryhard territory like a certain somebody. But I'll get to that later. Funny how her ace is the adorable dragonite. I like her hair too.
Charlotte - she's made one of the biggest impressions out of all the gym leaders with her memorable dialogue, cool backstory and infamously being of the hardest leaders in the game(moreso in episode 13-14). Her type matches her character moreso than any other character perhaps, and it's interwoven into her really well. However I must make a complaint: BRING BACK THE CIG. Her vs portrait was better with it. No one is going to accuse Reborn of promoting underage smoking because of it.
Radomus- this guy is awesome. Lots of personality, mysterious, charming, nice use of the Mad Hatter theme, and it helps that he debuts in one of the game's most memorable segments. This guy is totally cool and there's still a lot we don't know about him yet. My only complaint about him is that he's too easy in battle which doesn't feel appropriate for a man of his caliber.
Amaria - she has some serious guts despite her friendly personality and insecurities, and it's a pretty jaw dropping moment to see her bust out a high level Lapras against Meteor goons. I do dislike that she's with Titania if only because she deserves so much better.
I like all of them to some extent except for Terra and Titania. Terra just kills immersion and goes too far despite being funny sometimes. Plus she reminds me of Tinty tina from borderlands and one of her dialogue (the timmy one) sounds like it was ripped straight from tina's. As for Titania aka Edge Goddess, she's just completely unlikable and the epitome of tryhard. Actually I suppose i don't like Luna much either. She looks too much like Alice(or maybe it's the opposite), her battle is a bit forgettable, Iolia valley is one of my least favorite chapters in the game, she probably has the worst Reborn chapter overall and a lot of characters cream themselves over her making her more trouble than she's worth. I also suspect she's a bad guy due to being a dark leader and her obsession with living her life as a fairy tale. She seems very irresponsible and even selfish too.
Also I have to laugh at "murdering defenseless grunts." Won't someone think of the poor terrorists! Meteor deserve NO sympathy considering the people they hurt and the ways they went about it. Also Sapphira killed the ones that were planning on assaulting her home in the first place so while simon was sad Tara died I could never feel all that sorry for him. Meteor apologists FTL.