Good question, but System Shock 2, developed by Boston's own Irrational Games along with the now-defunct Looking Glass Studios, is certainly in the top 10, in my opinion. Now that Electronic Arts holds the rights to the System Shock franshise, though, we are unlikely to see another game in the series which could live up to my high expectations.
Enter Irrational's announcement of development on a new title, BioShock.
Allow me to pin my unrealisticly high hopes on you, little title. Please?
Man, I hope it doesn't suck.