This is important - assassin /type/ character, or Assassin prestige class? Because the Assassin prestige class does a piss poor attempt at being an assassin. You're much better off with a Shadow Hand swordsage, or a Ninja, or hell, even a Rogue.
There's lots of ways to go. Rogue/Wizard/Unseen Seer/ArcTrick, nets you all kinds of delicious spellcastery bullshit.
Thrower rogue, the ever-sucky melee rogue/ninja/whatever.
You could build a melee build (pouncer, chain tripper, dualhit/rip/robilar's/imptrip) and then give it hide in plain sight, hide, move silently. Ranger does alright at that if that's what you're after, ranger X/fighter 2.
You could build an archer swift hunter build, give it shot on the run or travel devotion, and then mainly focus on using hide to snipe with full attacks or Manyshot with lots of skirmish + ranger BAB goodness.
An interesting idea i've always liked is the Cragtop Archer PrC focusing on various things that add to single shot damage - splitting, Power Shot, skirmish, composite bow, magic, spell storing arrow etc, so you shoot a guy from a couple K away and drop him in one hit. Could do something similar with Spellwarp Sniper and one of the snowcasting blaster builds.
But yeah, if you want to stab things in the back, you've got a couple choices.
Thrower Rogue.
Self-buffer Rogue (Unseen Seer gish, or psywar/egoist selfbuffer/gish).
Dagger Rogue! (you suck hard)
Dagger Ninja! (you suck slightly less but still fairly hard)
Melee Build w/ Hide! (You're a melee build... with hide!)