Rogue seems like an obvious choice, but it depends on what flavor you want and what stat(s) you're emphasizing.
Ignoring mechanical considerations, I like the idea of an assassin/sorcerer or assassin/avenger.
You'd be highly versatile, able to switch off easily between melee and ranged damage, and lots of evasive capability.
Why are you planning to make an assassin hybrid? Because it sounds cool or because it fits the concept? (The assassin isn't a very well designed class).
Rogue seems like an obvious choice, but it depends on what flavor you want and what stat(s) you're emphasizing.
Ignoring mechanical considerations, I like the idea of an assassin/sorcerer or assassin/avenger.
Stat-wise, an assassin/sorc (Dex and Cha) should work very well. Drow or halfling would be the obvious race picks. You'd be highly versatile, able to switch off easily between melee and ranged damage, and lots of evasive capability.
If so, you could prolly do a good avenger/rogue hybrid if you focused on ways to get combat advantage when you're the only character adjacent to an enemy. (Isn't there a thief stance that does essentially that? If we get hybrid rules for Essentials subclasses...)