Well without knowing what level or abilities you have I can make some general suggestions.
You want to have a high Dex & Str. You will need to have a fair Con and Int as well.
Races that would work well are Elf, Human, & Half-elf (Skill focus for stealth)
Rogue talents to think about are Fast Stealth, Bleeding Attack, Surprise Attack, and depending upon your Str and Dex either combat trick or finesse rogue.
The advanced talents would be crippling strike and maybe opportunist.
You may want to go two weapon fighting and pick up rapiers or elven thin blades. Improved critical and bleeding critical would be good higher level feats as well.
If you let is know more details about what your level, abilities build, and race choices are we can get into better details.
If you have not given the new rogue a look, you may want to do that first. They have added talents at lower levels and they change things a bit. Also with the skill consolidation rogues are not as dependent on INT any more.
Some of the new feats you should look at are Wind Stance and Lightning Stance as they would work well with what you have described above. There are also several critical based feats like bleeding critical & blinding critical from the critical focus tree for higher level.
Sneak attack still works against flat footed targets anytime you are less the 30 feet away from them. You can use stealth anytime you have some level of concealment or cover. So a crowd should work.
Also check out this for more info on Stealth.
They added rules for using bluff to create a diversion so you can stealth in places you normally couldn't, and there are sniping rules that are difficult (-20) but great when succcessful and allow you to sneak from range.
Some of the spells don't interact with sneak attack the same way. Blurr and displacement come to mind, you can no longer use them to get sneak attacks.
Not really what your going for, but intimidate now makes enemies shaken, and there is a feat, shattered defenses, that lets you treat shaken foes as flat footed.
Rogue tallents allow you to do things like move your full movement speed while stealthing. Also allowing you to always get sneak in the suprise round, even if your opponent is not suprized and went before you.
You can now get sneak attack against almost any monster. Gone are the days of undead and golem imunity. Now only things without discernable anatomies, like oozes and incorporeal creatures, prevent you from sneak attacking.
The Shadow Dancer prestige class still exists, so you might want to take a look at that.
But, from your character concept, I would suggest the Lion Blade prestige class from Taldor: Echoes of Glory Pathfinder Companion.
Providing you don't mind the requirements (you'll need the Inpire Competence special, aka at least 3 levels of Bard, and Taldor Affinity, thus a taldoran background, having been trained in taldoran assassins schools and probably still in service of Taldor) and the lower SA bonus (+5d6 at CL 16 instead of +8d6 for pure rogues), I believe this class would satisfy your "Strike fast and disapear into the crowd" concept.
To summarize, this class gives you the Hide in Plain Sight ability (like the shadowdancer), makes you a master of disguise (removing penalties for changing sex, race or gender and such) and makes you move faster and easier into crowds (remove the movement penalty for moving into a dense crowd, which normally counts a rough terrain AND give you a large speed boost when in crowd a few times a day). Plus a few other abilities well suited to the spy/assassin type.
Edit: The class also include huge stealth bonuses and some kind of special dodge ability, which give you partial concealment for a short time, event out in the open.