We played a number of assassin types over time in RQ3...
An elf makes a decent assassin with Camouflage, silence sphere, chameleon and reflection, and cult skills like hide, sneak, and climb.
We house-ruled that Black Fang gave poisons and a poison-proof mask to initiates, and they used hidden hand blades (à la Assassins Creed). I actually played a Black Fang assassin who killed the rest of our party in their sleep with her poisons after they had all attuned to an orb that gave them insane chaos features/powers (her current mission was to kill chaos creatures for bounty).
Eurmal is great (especially if he visits new shrines), with illusions, Strike, invisibility, hallucination, and whatever other weird spells he can use.
Yinkin/Orlanth has so many tools he can be good at anything. Especially with spells like flight and leap, plus teleportation/guided teleportation from Mastakos, he can get in and out of pretty much any situation.
Similarly to an elf, a hunter could make a good ranged assassin, with a good sneak, hide and track, and spells like sureshot and multimissile.
Maybe a combination of Uleria and some other god could use its seductive powers to get close.
We also played with ninjas, orcs (small and sneaky, but super strong, and with sorcery spells), and others I'm sure.