What do you like playing? Do you want an ADC (ranged AD dps carry)? A mage (AP caster carry)? A tank? An assassin? A versatile champion? An OP champion? Without that I can't tell you much more than an empty list of things I like :P
Fizz is a really easy to play, always sort of OP assassin. ADCs feel quite balanced lately, so there isn't a clear "OP" ADC. Vayne is weak at the moment but she'll get a small buff next patch. Even if she's on the weak side, it's a great pick with huge potential to solo carry some games.
To answer something: try Fizz, he's super fun.