These are only a few examples of typical assassins. While there can also be Mages e.g. Ahri that can be considered as an assassin, but still has Mage as their primary role.
Not an expert but: Diana is relatively easy to play as you only need to farm up till you get 6 and then just constantly roam when your R is up. And you also have the ability to play well in early game, worth the risk is the question.
Taliyah is good for roaming but not assassin, good to counter assassins
Akali if you know how to play her well and snowball with her.
AP:
AD:
These are only a few examples of typical assassins. While there can also be Mages e.g.