You asked for my honest opinion, and that includes rude comments. Sorry.
Vanessa- She is pretty, and has a good voice. In HSM her character isn't silly, but when she guest stars on Suite Life she takes on a new personality as quirky Corrie. And as I stated earlier, she can take on various roles, each with different personalities, and you wouldn't know that it was the same person if you didn't see her face.
Ashley- She's also pretty, her style fun. Her voice slightly strays from what I'd consider a beautiful voice, but that's only because her valley accent laces her singing a bit too much. She's funny and serious as an actress because in the Suite Life her character is a smart and funny person with a wacky best friend. And those kind of friends always cause predicaments that make her the hilarious person Maddie is. Sharpay, the mean girl, takes on a whole different air as the scheming, evil, selfish opposite of Maddie.
Destiny- In my opinion, the only thing she has is a pretty face, but if she messes it up with makeup like she did at the D C Games concert, well, that goes as well. Her voice isn't that great. If you listen to her sing plainly, there is a strain and hoarseness to her voice. In her recorded works, you can still hear the strain and retainer. I think her "Hannah-voice" is as fake as her name, which is only a nickname. Her real name is Destiny Hope Cyrus. As an actress, either she is no good or her life is an act. She is a bit too masculine for her Hannah character. When she said "Sweet mama, it the Jonas Brothers" it sounded a bit like a man cheering on his favorite team on Monday night football. I do not think that she's funny, because even though her script is funny, she doesn't put feeling into it. She just simply says words and does actions they tell her to do. The true reason she became famous is because of her father. Honestly, he was on a television show in the 80's and brought her on for one episode as a cameo. If you do not believe me check on Wikipedia. This is my opinion on things.