(On a sidenote, have you noticed that 'shit' is generally bad, but 'the shit' is generally good? Lissa once jokingly inquired if 'ass' could be made to be a good exclamation in the same manner: 'the ass'. I've introduced the idea to a few people and we use it occasionally, but it hasn't seemed to gain any widespread acceptance.) :P
Many simply come and go. Some others fade in general popularity but linger around and are still used on occasion. However, one that seems to have stood the test of time is 'cool'. Cool has always been cool as long as I can remember--and I'm sure much longer than that as well.
Cool is probably what I use the most. Otherwise, I tend to like ones that aren't really fashionable: groovy, spiffy, nifty, etc. I could claim I'm retro, but I'm probably just out of touch.