I swear that to this day I don't get what Gygax was thinking with the Ranger. Actually, I wouldn't even have based the Cleric's divine powers on a spell system, but that's dragging this thread way off topic.
AMEN!
I could see someone using certain base classes as assassins- Rogues, obviously, but also Scouts, Soulknives or Rangers.
(really, under the right circumstances, any class will do)
If you have AU/AE, the Akashic can make a credible assassin.
However, for a Pirate-themed campaign assassin in a with minimal mystical abilities, using only WotC material, I'd probably multiclass Rogue/Fighter, Rogue/Soulknife, or Rogue with 4 levels of Ranger...taking a Parrot as the animal companion.
I saw an African Grey in a bad, bad horror movie- it had all the best lines. Whenever it sensed the supernatural horror stalking the forest rangers, it said "Uh-Oh!"
Despite this, the idiot rangers kept trying to save it by leaving it behind.