While I enjoy my retro gaming lets be realists, there's plenty of crap titles to be had for every decade. Way back then you could easily play even the badly ported games and still enjoy it as if it was the real experience because you didn't know what you were missing.
Looking back now its easier see the whole selection of games and pick the best so if you can't find a worthwile game of the current gen to hold your interest you just go back to what you remember being good back then.
But to keep it fresh you have to start looking for those hidden gems and even explore different genres like I did with Fallout, a rpg thats not boring?
So I'll give you a couple of my favorites that went unnoticed by the great dumb majority:
System Shock 2 DEMO
Thief: The Dark Project DEMO
Give them a chance to suck you in and I'll almost guarantee you'll want to hunt down and buy the real thing.