Which of these 4 champs should I buy?

So, I have 4800 IP and I'm looking to buy a champion. I main diana, so I'm used to doing a lot of damage and carrying and jumping in fights ETC.

So the 3 that caught my eye are Cassiopeia, Irelia and Orianna. I want a champion that can just jump in and rape. I also want a champ that's fun to play. Irelia seems to fit my requirements, but she's a top, and I can't play top.

Opinions appreciated!

Edit: I just want a champ that does good damage. And please understand, as a diana jungler, "damage" to me means very little unless i can kill someone while they're at full health, like I do with my diana combo.