Kinda depens on how you like to play, Akali is an insane burst-caster, and pretty much devours even build-completed squishies. However, she's not useful to pushing lanes and unless you really coordinate your kills with team offensives, you'll be contributing nothing to your team. This is especially true in normal games where I've seen Akali get over 15 kills with zero deaths on the losing team... more than once.
Kat really isn't a ganker. She can fake it with a Rylai, but despite the looks, she's a caster. She can be built well to push lanes, or hop in after initiation to do terrible, terrible damage.
Shaco is similar to Akali, in that he can absolutely eat squishies, but he's more about positioning, ambushes and baiting than OMGBURST. Unlike Kat and Akali, he also has the advantage of being able to take out towers pretty well. He's also a pretty popular pick in 3v3 if that's your style.