Hmm, I had a two guy team of assassins detailed in my sig. One sorcerer (for firepower), the other one a chainmonkey for battlefield control and a little healing.
As a single char, I'd go warlock, wilder or psio. Even better: Lurk. The lurk has nearly the rogues sneak attack but he can get more sneak attack damage than a rogue by putting PPs into his Lurk Augment thing. All he needs is Ranged Lurk Augment feat.
Put 4 PP into your Lurk augment and at level 4 you'll have a sneakattack with +4d6 damage. Use a handcrossbow with Rapid Shot, +4d6 with the first attack and shoot him again (sneak attack +1d6). Go human for 3 feats. And you'll have all the skills you want plus some additional psionics powers. Skate is great for escaping, Precog and Prescience are good too. Stygian Weapon is great, no save and you might stun him.
That only works within 30ft though... so the singleclass ftr sniper shown above can't be beaten. He is well advised to prepare escape routes and have some horses waiting for him...
Forgot to mention: A bard makes a wonderful assassin. Spells to disguise and social skills to impersonate someone else, more skills to get into someones bedroom at night... CDG with greataxe.