They all play super differently. Vi has a very "in your face", commit commit commit gameplay style. It's best served by working with someone else who has a form of hard CC - she's not a solo monster.
Diana is more about lining up your commit and leaping in and keeping someone from getting away. Her kit lends itself better to being able to murderface a lone person.
Rengar is also designed as more of an assassin, but with the best escape capability out of the three. He's also able to sneakily start a fight, so there's that too.
Which sounds most appealing to you? Go with that one!